Some people are picky or fussy eaters and will be throughout their lives.
With young kids (and gender is really irrelevant), sometimes it is sensory (eg textures/taste) and sometimes it is psychological (negative perceptions about body image, a parent constantly on a diet, being  told they are fat, etc.)
Psychological issues should be addressed as they can be extremely damaging, sensory issues = not much you can do about it.

If you are interested, you should find out why your SS is a picky eater - and possibly address it.
Eating starches and refraining from fruit and veg and can be that he eats what he is used to or normally given.
In that case you can expand his palette by introducing new foods one at a time.

I have always been a picky eater and it is not going to change.
In my case, I don't like processed food and never did.
I also tend to look at how clean a place is before I will eat there or if food is prepared on site - if not, no thanks.
It used to drive my aunt insane when I stayed over with my cousins because food was always an issue - in her case she was a rotten cook, but hey ho, kids can't or don't cook for themselves and adults complain...

Unless this boy is desperately thin, I suspect he fills up with junk when his parents aren't looking - or at his mother's house?  Personally, I adopted the approach that I put a good meal in front of my daughters, from toddlerhood onwards - if they didn't eat it there was nothing else until the next meal.  But then I am old school and not in favour of pandering to faddy eaters. Either way, it is not up to you to solve this lad's food issues.  I would leave it to his bio parents to sort out. 

Some people are picky eaters for their entire life.  And some 12-year-old boys are 4'11 and others are 6'2.  Most haven't gone through puberty yet at that age, and for some boys, their growth support doesn't come until 15, 16 or later.  I have a nephew who didn't hit puberty until 16 and now he's 5'10.  DH is 5'6 and SS is 6'2 - so a parent's height only matters so much when it comes to a child's height.  

Just let it go. 

Read my blogs and posts about this.

OSD, now 16, I think has "Selective Eating Disorder."  It's related to autism they think.  She also is very, very picky about what clothing she wears, touching and sounds.  Does your SS react to those things as well?  NO fruit is a major sign of this; most picky kids will eat some kinds of fruit.  OSD eats no fruit, no veg.  Her diet is pretty much carbs with the occasional white fish or chicken (sometimes shrimp) - mostly white foods. She, I think is vitamin deficient - dry hair, bad looking skin, always tired.  BUT there was nothing I could do because parents didn't want to deal with it.

YSD, now 13, was also very picky at first, but she's now eating pretty well but for some very wierd food "fear" she's developed for certain things - luckily avoidable for the most part.  I think she'll get over that as she gets older.

To identify all this, when they were 10 and 8 I lost my mind over the food issues.  Look up how to introduce food to toddlers and picky eaters.  There's a lot of info on the internet about this.  Do family style meals in which everyone can serve themselves what they want, don't admonish if they don't want to eat something, but they have to TRY everything.  It can take 3 or more tries of a new food for a picky kid to really decide they don't like it.  If they don't like it they MUST explain why - not just say 'I don't like it' - is it texture, flavor, spicy, chewy, color?  (YSD reacted really well to this option) A normal picky eater will expand their food choices normally over a couple months, a food disordered kid will not (fights, metldowns, spitting food out, wanting to throw up if they eat something they cannot process mentally...).  YSD developed out of picky eating quickly, OSD never did.  Her food choices are the exact same as when she was 9.

This will cause issues when dining out, if you haven't encountered this already.  I do not help any longer at restaurants. A picky eater will have to learn the skill of deciding what to eat and how to order when "there's nothing on the menu to eat."  At 12 you can help them a little (your DH) but they shouldn't rely on adults to decide. My god, DH and others still helped a 13 and 16 yo order this year - but when alone with me, SD13 does it herself! 

The skids are notoriously bad eaters. They used to say they were allergic to the oddest things just to get out of eating. There is a friend of theirs who is even worse. Sounds impossible but it’s true. 

Personally unless they have other issues, I think this is a result of coddling by the parents. I watched DH wait on them and negotiate and all sorts of servile behaviors toward them. They were picky because they were allowed to be. 

The kids friend and a coworker who is also super picky have parents that supported the picky eating. 

My nephew was a picky eater when he was little, but the pediatrician always told my sister that he was still growing, healthy, so it was "enough." Today at 17, he's still not the most adventurous eater - he has his favorites and it's not that different than when he was a kid - pasta, anything cheesy, and now hamburgers (which he never used to like.) But yet, he's really tall, his dad takes him to the gym to work out, so overall, still healthy. I don't think they ever really grow out of it...they have to be introduced to, and get used to, new foods. I have coworkers who tell me their husbands never ate XYZ until they met them. My BFF has been giving her toddler tastes of all kinds of foods since she was young and she's a GREAT eater!

At any rate, unless it bothers you personally, relax and let the bioparents deal with it! 

actually got pickier at that age. Well, two of them did. the oldest and middle girls do not eat red meat, pork, or seafood. They simply don't enjoy it. They never really did I guess but ate what they were served prior to that. They also don't like certain textures and I am pretty sure once the middle one is out of the house she will become a full vegetarian as well. She eats chicken now when I make it but doesn't really like it. 

My oldest and youngest only have a weird thing with squash. They hate it. All types of squash. It's weird. 

Is he on any medication? My godsons are on ADHD meds and hardly eat anything. Although my SD is the same way and she’s not on any meds. I thought she would grow out of it but she’s 11 and still going strong. I just gave up. 

I am a working mom and in our house everyone eats what I cook, I do not have time to cook something different for each person. In the evening I usually have vegetables and chicken or fish and a bit of pasta. DD eats everything and if she doesn’t, well, she is not hungry. One can always chose and have vegetables only, or meat only,  or pasta. In addition, I always have fruits in my house and provide DD with it for a snack. 

Now the DH’s kid eats only French fries or pasta and nuggets. All of that with the commercial bbq sauce. He doesn’t eat veggies or fruits. For desert he has commercial cookies and cakes. 

Welll I do not and will not buy any of that for my house. I do not want to teach my DD to get used to processes shugar and all chemical conservant that they use in processed food. 

In our house we do not have and will not have any of that. 

In the beginning I tried to explain to DH that processed food is very unhealthy, tried to feed his boy with some veggies and fruits , cook a home made tomato sauce etc. DH told me he understands and agrees but the BM is eating exactly the same food as the boy does (she is way overweight btw) and the ILs cook him the same so he doesn’t see how he could reinforce any eating habit. 

I gave up and this is why I leave my house with DD when the boy is visiting. I do not want to explain my child why she has to eat fruits and veggies while the boy is eating chips and nuggets with bbq sauce.