No worries with arm strength. My son never threw with a full pitching motion, he kinda brought his arm halfway pack then threw, hence giving up some speed. Finally this summer at 14 he’s finally come around to a full motion. He hasn’t had a pitch lesson in years btw so this has been a natural progression. He was always the 3-5 best pitcher on his teams and now he’s moving up the depth charts for pitching. This games start to matter more going into hs than 12u and under. So really no worries imo.
Throw a football with him, basically a weighted ball, thats a great fun way to build arm strength and sprinkle in some long toss.
Another awesome pitching option is the Mustard App. Created by Tom House. Its free and has arm care stretching videos and more. You can video your sons pitching motion and forward to them for evaluation and it tells you what to do for areas of improvement. Its a must have app imo.