Yes, it may be harder because he is older. I saw a boy about 6 years old there at the same time, and he was drinking a cupfull. The father was bribing him with treats when he was done. Is there anything you can do bribe him so he’ll drink it?
Actually, given his age, I would call the department that’s going to be doing the test before hand and talk to one of the nurses. Tell them your concerns, and ask them if there’s anything you can do to prepare him or that they can do that day to help him to drink it. Maybe they can add it to something.
Good luck.