Ur doing a awsome job, that kid will always love u more than his biological father. It takes a real man with a big heart to do what ur doing. Hats off to u brother. God bless u.
Let the kid play video games some. Helps with hand eye coordination/ timing motor skills ect. Also a boxing speed bag set up, amazing for coordination and timing. Start with a big bag then move to a smaller one over time, mabe jump rope also.
I have 2 kids around that age i have to make them exersize, when I was that age u couldn't keep me from physical activity I usto get high off it. I still do. Kids are different these days. Try to make it fun for him and offer some type of reward for after he finishes, my kids look forward to a big glass of chocolate milk after a workout.
Focuse on Mind muscle connection it's hard to grow a muscle u cant feel. Ur spot on with the bands mabe get a set of light dumbells, 2, 3 , 5, 8 ,10, 12 and 15 pound dumbells. I would end the work outs " once hes a little sore" end with a round of fully flexing and extending every muscle in his body that he can feel the muscles working then followed by a light strech, u also mentioned grip strength get him some of those grippers or squeeze balls with different resistance.
U have him pretty active last thing have him on a good balanced diet , 3-4 meals a day with a good amount of protien. .8 per pound ish, So he has the building blocks to recover.
I'm no expert but this would be my approach.