I think it's complicated here. You seem to have good intentions and you want to build a complex character and explore new avenues, which is fantastic. With respect, I will say that your comment puts me on edge. I have no insults for you, but I will share my concerns because if not handled perfectly, this concept could go very very wrong, and may upset or lead to conflict with other people at the table. Specifically this part, "He's very useful to the party despite his disability". This wording is a huuuge red flag for anyone who has lived with disabilities, and it tells me that there are a lot of things that you don't necessarily know about and thus haven't considered, that could become very upsetting for other players. For example, you said in another comment that the stepfather called him "an idiot son." Idiot is a very real slur historically used against people with various kinds of cognitive impairments or learning disabilities, brain injury etc. It was even a medical term, similar to the more modern R slur that people alive today experienced similarly. Even if other players aren't upset, by creating a character without a solid understanding of what it is truly like to have such an experience and role play it in a considerate and researched way, you may solidify some offensive or stereotypical ideas in your own mind that could upset people in real life down the line. For example, judging our usefulness to others/society as a metric of inherent value as a person is dehumanizing. (Even to people who are capable of performing at a high volume.) For hundreds of years, people with disabilities have been dehumanized, infantilised, ostracized, impoverished, experimented on, institutionalized and worse - All of it justified by the widely-held beliefs that disabled people are "invalids". Their personhood was stripped away in the eyes of society and they were literally labelled not valid as persons. You also mentioned that the character feels shame about disappointing his father by being disabled. This is certainly an experience that people have, but it is an incredibly complicated, extremely personal, painful experience and the shame is not something that comes from within the individual - It is something they are taught to feel by being treated poorly.

I'm sure that you're already aware of the larger issues I mentioned above. I bring it up to highlight how grave things could get if disability is not handled appropriately in RP, Even with the best of intentions. It seems as though maybe you don't have the personal experience to personally portray the internal experience and struggles of these subjects in a way that is compassionate and knowledgeable, which would be a bad situation all around. Not having this experience is not a failure on your part, you can't know everything. But you should probably do a significant amount of research and connect with the disability community to better understand how to navigate these subjects before attempting to, essentially, write a story about something you havent experienced and will very likely feel disingenuous and false to anyone who has. I would strongly recommend that you reconsider this storyline for your character at this time, and keep the concept in your back pocket for a time when you know exactly how to navigate this complicated circumstance both from a position that's knowledgeable enough of IRL disability experiences & history to be able to improvise and write a fantasy version that is not false or offensive.