1. Have him fly with another instructor, just to verify what you're seeing.

2. Sit down, give him the honest assessment which is "no end in sight, no way to predict it".

3. Give him the option of continuing, with the understanding that there can be no expectations or timeline. Basically turning avgas into noise and see what happens, if that's what he wants to do. If you can't keep the training safe (ie really sketchy landings), don't give him this option.

4. Make it clear to him that instructor shopping might get him a rating, but his lack of aptitude could get him killed.