My son is the shortest boy in middle school and struggling

I have two sons a pre-teen son who is on the small side and a younger one who is going to be 6’+ when he’s older. My oldest is struggling hard core right now. He is the shortest boy in the middle school at 4’7” but of course has a personality larger than life. He loves playing sports but is bullied for his size and made to feel insignificant. On the home front he is made to feel insignificant while at his Dad’s. He asks his Dad if he can spend time with friends and his Dad’s reply is “we’ll talk about it.” I have him in therapy but he broke down tonight crying that no amount of talking to adults makes it better. He talked about how other kids don’t have a parent that beats on them and why can’t he be normal. I’m heart broken. I have zero proof of his Dad hitting him but his distraught is wearing him down. I could use any and all advice. I’m at a loss as to how to help beyond continuing his therapy sessions.