and where is the stretch or feeling of restriction - anterior, posterior....?
tight lats would be a possibility if he can get into the shoulder press with elbows low but not when he pushes up through rom.
the position requires shoulder abduction, lateral rotation and horizontal extension. work out which one is lacking and then we can work out whether its more likely soft tissue or joint and which structure exactly following that. Then you can try a strech which targets the problem
