I had a similar issue years ago, I had the knack of getting all the problem students from other instructors. One of my student started when he 75 years old just wanted to fly. Guy waited his whole life and was finally able to do it financially. The guys passion and love of flying was remarkable.. made instructing very enjoyable.

After about 50 hours and still no solo I sat him down and explained I was willing to keep flying with him but there is a very strong chance he would never get his private or even solo as well the amount of money he would spend. He agreed and kept on flying and did solo but that's as far as it went.

My suggestions would be:

Have him fly with another experienced instructor for a lesson or two, no offense but I have seen students do much better after a change of instructors. Some people mesh some don't.

If he still doesn't progress give him an honest evaluation ... don't sugar coat it but be professional. If he wishes to continue and understands he may not make it and also understands the cost then keep flying with him if that's what he wants but be firm I wouldn't want to see a guy throw thousands of dollars away.

Best of luck