Left handed pitching velocity

Hello…My son is a 19 year old left handed pitcher. The past few years he has gotten much bigger and stronger but oddly his velocity is not increasing. He currently throws the same speed when throwing with 75% effort versus 100% effort. I suspect he is tightening up when trying delivering a pitch at 100% which is taking away velocity. It’s possible he has a mechanical issue but I don’t believe that is the source. I took a video and I don’t see anything mechanically that would be causing this but I am not a trained eye. Thought I’d ask if anyone else may have run into a similar problem and if their are any recommendations what to look for.

Yes, this is common. I was once told by my coach to hold the ball no harder than an egg. It takes a lot to get used to throwing that way, if he’s been tightening up for a long time. It’s very difficult to have a loose arm and a tight grip and vice versa. I spent a lot of time throwing into a net at first to get the feeling right and being able to have the ball go where I wanted. Fortunately, I was only 13 at the time. I was noticeably faster after that and, in the outfield, I was very productive at throwing runners out trying to tag and advance. In middle school I even threw out two runners trying to score from 3rd…in the same inning. What a rush!