I can't make him cum:(

Okay so I've been fucking this guy for about two months or so now and I can never make him cum, I'll ride him for ages and suck him off, everything nothing works and I don't know what to do cause I feel so bad everytime even though he tells me it's fine:( is it me? I know he takes medication for his knees and he's got like arthritis even tho he's only 19. Could it be something to do with that? Please help me:((((