It is not unusual for people to feel uncomfortable and shut down as they go into ROM they have neglected to use and lost strength from. That is a training problem, corrected by using the ROM and developing strength.
If a person has developed actual pathology in the joints - bone spurs, for example - no, you don't push into that defect. Bony changes cannot be fixed by training and smashing soft tissue into them is a bad idea.
That said, it is not clear if your post if you are working him into normal ROM or something beyond that. Why are you trying to get him to "extend further" on EZ curls? Further than what? Reach higher into pulldowns? Why? Relaxing and letting the joints/ligaments take the load is not the idea for anyone. Hypermobility is not a good thing. Not sure that this is what you're trying to get him to do, or what is actually going on. Perhaps you can clarify for us.