I know that this my not be the most popular approach, but I would insist that he attend. My second oldest wanted to skip her graduation. I told her that the graduation ceremony was not for her, but for me. I did not spend 18 years looking after her every educational need, only to miss out on the "prize" so to speak. I also explained to her that my life revolves around my children - I do so much for them. However, when I ask for something really important to me, they need to pay attention and do it, since it isn't very often.
I did not offer an incentive for going to the graduation, but I did do "disincentives" if she didn't go. No graduation means no party, which means no gifts (from me or my side of the family). I also told her that if she didn't attend, that she could not ask me for anything in the future (of course I would feed and clothe her), but no extras like cell phone, gas money, etc.
Finally, I did explain to her that being an adult includes compromise. Up until that point, I had not expected much compromise from her - she was my child. However, graduation is the start of adulthood and the start of learning that everything is not about her wants and wishes, but other peoples as well. That's what make people good adults, good parents, and productive members of society.
Whatever you decide, I wish you luck.