I'm convinced there is only so much you can do with arm strength. Think about the kids you saw at 8U and 9U who had the strongest arms. 95%+ of them still have the strongest arms now. Understand that large increases are usually not realistic. And some of it is also time/growth. I'd spend some time looking into his mechanics. It's OF velo, so his crow hop might be tweakable and net him some gains that the weight room won't. My 2021 is right at 5'9" 160. His OF velo at PBR last August was 85. At an October PBR is was 88. February got to 90. Those gains were noticed by almost no one so there's one example how little it mattered to prospective coaches. Almost no lifting was done over that period. Casual lifting, at best. In my mind, I think it was the mental piece that allowed his velo to rise like it did. He's continually fighting getting noticed as an undersized guy and he went into those PBR events on a mission. He wanted it BAD. But again, the 5 mph he gained in 6 months didn't result in any returned emails or calls, so I am not sure I'd put much time in to it. Just my 2 cents.