It sounds like you are another student in the school (when you said he's like an adopted little brother). You might want to gently touch on the subject "with him". Just mention to him, that you noticed he was having problems with letters in words & was wondering if anyone is working on helping him with that?

If he says yes, then great, you can help with reading with him, but if he says no, then you might mention to him that there school does have help available that can make reading much easier for him & more enjoyable. If he sounds interested, suggest talking with his parents & then have them talk to the school about getting him into a program for help so that he doesn't continue struggling with school. If he's not getting the help that he needs, that is really awsome that you can be the one that can possibly get him the help he needs for his future success.

Great that you are that observant.....that is very special....it's wonderful that he has an adopted older sister kind of person like you ....who cares,
Debbie