pitching analysis video
Releasing on a bent front knee is fine if he has the leg strength to firm up and brace - which he appears to have. I also don’t think stride length is an issue.
I do think he has a sequencing problem in that it looks like hips and shoulders rotate together which minimizes hip and shoulder separation. In other words, he has early shoulder rotation, IMHO. And I’d trace that back to inadequate 'equal & opposite". He appears to get to an ‘equal & opposite’ position but vacates that early by swiveling the glove arm before front foot plant. Try to get him to maintain the glove side in an ‘equal & opposition’ position until front foot plant. That should let him keep the shoulders closed longer while the hips open up.