Are you needing to book hotel meal etc, if so , that would need to be done early as obviously with a graduation everywhere will book up fast, so, could you use this as a means to force his hand to either confirm or decline his invitation. If its the latter then he has to man up and tell his son and step grandson he won't be attending, up to him whether he gives reasons for his decision. Of course you still need a room if you go without him so maybe that won't matter so much!.
If its any consolation my husband is similar , in that, I know if an invitation comes through our door, the histrionics that accompany it have to be seen to be believed ! He has no social skills at all when out in company, I sometimes think he has adult autism , and I am being totally serious and not in any way being facetious . He just doesn't know how to do social niceties and he's 70 so has had plenty of time to practice !! Its become a standing joke now amongst friends how he gets on but I don't always find him amusing .
I sincerely hope things resolve Greyduster and most of all I hope you have a great day celebrating your step grandson's amazing achievements . Take care x