Could be. Very often, when a pitcher is overthrowing, his release point tends to be late, and when that happens the pitch is likely to come in high. I’ve seen that happen often enough. I don’t know what the kid’s arm angle is—his arm slot—but often, when he’s throwing from a high arm slot (overhand or high 3/4), is when this problem is most likely to occur. Let him try dropping the arm slot a little, say to a regular 3/4, and see if this doesn’t correct the problem. And above all, he needs to relax a little. After all, he was facing his old team, and he knows them—so he knows how to pitch to them.
And if he has a changeup in his repertoire, let him use it a lot more than he has been doing. Remember what Babe Ruth, who knew a little about pitching, used to say: A good changeup will cause batters more grief than anything else. And if the kid doesn’t have a changeup, he should develop one. I recommend one of my favorites, the palm ball, which is easy to throw (you use a fastball motion) and very hard on the hitters. otherwise, all in all he should be fine.