Have you told him that you liked doing it because you liked spending that time with him? If it's something he's now only moderately interested in and doesn't want you wasting your time he might change his mind when he knows that you just like the experience with him. Also, when you put money into something like that, he has to understand that it is a commitment. Possibly you could request that he just gets though his next grading before he quits so that he wouldn't have wasted the time since his last test. He may get back into it and remember why he liked it in the first place.