I agree with others that you need to consult a trusted trainer. I would start with meal prep. A bunch of boys would do it on Sundays and make steak, chicken and potatoes or pasta to put in prep containers. They ate a meat/carb meal for each meal, even for breakfast, then ice cream or a shake after dinner. They all put on weight. 125 seems low to start anything with weighted balls. When my son did driveline, he did the offseason workout and the recovery. He did gain strength and velo but he was at least 175lbs when he started. I would focus on weight room and diet until he can add some mass. He can throw everyday while doing that but his gains will start with weight and muscle.