Most car hire/rental companies (around the world, in my experience) will not entertain a driver who is under 21. I just had a quick look at some T&Cs for the the dozen or so I've used in Spain over theyears and no-one will allow a driver under 21. You will also find they often have a minimum amount of time that any driver must have held a full licence for (2 years) as well, irrespective of age.
That's life I'm afraid. Maybe some small back street local car hire place where ever you are headed might let him drive, but I would suggest that's your only chance.