Bioparents 90% of the time are willfully incompetent......their spaws reflect this

His BM might be smart but she sounds like she was too ignorant to help him be smart too.

Is he mentally challenged? Has he been diagnosed with any cognitive functioning/mental disabilites? Is he like this everywhere in life (at school too)?

Do you make him repeat back what you say to him?  It's easy to fall into the lecture trap  but I found making mine repeat back to me what I said clarified the message.  If they got it wrong I said it a little different and had them repeat back to me.  Forcing them to think.   
 

Now if your SS really did fall off the turnip truck head first, this might be frustrating for you to do with him but I'd try it if you haven't already.  He probably shuts down mentally automatically now. Every encounter is stressful and highlights his consistent failures.  

For my SS, though his mom and I were constantly engaged and parenting, it took his 7th grade band director to get through to him. 

"There is nothing special or impressive about you."  

After this message SS came home all upset at what the Director said to him.  The message was nailed home when his mom and I did not get all butt hurt for him and call the school to complain.  We sat SS down and asked him what he was doing to convince his Director that there was nothing special or impressive about him. We reviewed his grades, not bad but definately not special or impressive, etc....   

That was when SS started to gain some self introspective and started to see that his choices impacted him, what others thought of him, etc....

Hearing it from someone other than family, broke the seal and got him moving down the path to start holding himself accountable. It took another 5+ years to have it effectively internalized, but... that is when I think he started to get an inking of clarity.

As for your 9Yo's, not unusual for 9yo boys.  Could be early onset of the teen boy brain farts.

SD here looks at dirt and it gets on her clothes. Food, mud, everything. It's annoying as she is also 9 and needs to learn not to get food all overself. I think she does this for attention and a reaction. She knows I don't like socks worn outside without shoes.. but does it. I tell her to not wipe her mouth on her sleeve.. she does it anyway. It's annoying but now I don't give a crap if I wash her clothes and the stains don't come out. She doesn't care, DH clearly pays no attention and I'm over being the one that cares. 
I used to buy SD new socks and underwear now I don't. She wears them to BMs and come back in SS underwear and socks if she isn't wearing BMs rags back! I have no advice other than what I do and just vent and disengage to save yourself mentally. 

Sounds a lot like my SS12. He's not stupid, but seems utterly incapable of problem-solving or even the most basic thinking. For three years now, DH has been dealing with SS and clothes. He'll bring down his laundry basket. DH does laundry, puts clothes back on top of the basket and has SS take the clothes up and put them away. Not only would SS not put the clothes away, he would dump dirty clothes on top of the clean ones, which would turn back up at the next laundry time. DH tried having SS take clothes up a few pieces at a time. He talked. He explained. He removed privileges. Nothing works. We'd make him do his own laundry, but we can't trust him not to flood the whole downstairs. I'll have to tell him the "repeat it back" method.

Anyway, in his case, it's a mix of self-centeredness, lack of natural consequences, being coddled/babied and just general tween boy-ness.

Sounds like you're handling it well. I don't get involved often. On the rare occasions I say something, I usually give him a look and ask "so, do you think that's a good idea or a bad idea?" When he responds "bad idea?" I just say "you're right." Course correction. But, a day later it all happens again.