There is not much to be done with infrequent visits. Your boyfriend can make sure to talk about you in positive ways and counter anything bad that the son says about you or answer any questions that he has.

I had a long distance relationship with my husband before moving to be with him. I did not really bond with my sd until after I moved in and even then it took time. We had nice visits and she seemed like a good kid but real bonding takes time and being present. Even now 6 years later it is a continual process when the child is not your biological child and there is a jealous BM in their ear. 

 I would not worry too much about the boy right now. I would though be making sure that your boyfriend is on the right path to gaining some independence from his parents and working towards being able to support himself and his son. 

Make sure proper boundaries are in place before you think about moving to be with him. For example I made sure that my husband was divorced from his ex before I moved (seems obvious, but people don’t always think these things through). I asked my husband to transition his daughter to sleeping in her own room, he understood (he wanted us to have some privacy too) and worked hard to transition her. I carefully paid attention to my husband’s money habits and financial situation so that I knew what I was getting into. I read a copy of (both) his divorce decrees. I paid attention to how my husband parented his daughter. I had different expectations of politeness than my husband but he respected me and asked his daughter to talk to me in ways that I found polite. I told my husband about how being around him with his daughter sometimes made me feel jealous of the attention she got while I felt neglected, rather than tell me I was silly to be jealous of a child he listened, understood and adjusted his behavior to ensure that he paid attention to both of us.   I saw that he could keep his house in good order that he would not expect me to do all the housework. 

While you are long distance it is a great time to really talk and get to know each other and watch carefully from afar to see how you would fit into this situation if you move. 

Unfortunately there is no way of ensuring that the BM does not PAS your bf's son.  PAS means parental alientation syndrome https://en.wikipedia.org/wiki/Parental_alienation_syndrome

This happened to me big time with the BM in my life, my SDs were 5 and 7 when I met them, and she told them all sorts of untrue things about me, and encouraged them to hate me.  As a result, I never managed a functional relationship with them, and probably never will - they are in their 20s now.   I suggest you proceed very slowly and cautiously with this relationship - I would give it at least 2 yrs before you consider (if this is an option) moving to be with your bf, and only then if he has his own place.  If you do move in with him, retain your own place for at least a year, in case it proves to be not what you want.  Spiteful BMs can wreak havoc in our lives, and even cause mental illness. 

Other than keeping the facts front and center there really isn't anything you can do to mitigate the toxic BM.

Keep the SKid fully abreast of the facts in an age appropriate manner. If BM lies, tell the Skid that BM lies and make it clear what her lies are and what the truth is.

Lather, rinse, repeat.

Kids need the facts to be able to protect themselves from the lies and manipulations of the toxic blended family opposition.  This becomes increasing critical as the Skid age and then transition to adulthood.  Defending one's self from a manipulative parent is a life long struggle.  Toxic idiot parents don't just stop being toxic manipulative idiots.

You should be most concerned about the BF's financial situation. Lives with his parents and yet doesn't have money for an attorney? Has a woman coming to visit and has her stay at his parents' house? Nope. Unless he had that 7-year-old when he was 15, he should be more together. (I'm writing this as someone who wasted over a year dating a guy in a similar situation; trust me, financial insecurity is a big red flag.)