If it was like my sister getting married, definitely the plus one needs to make an effort. As its your cousin, they are not going to miss your boyfriend not coming. His friend is getting married and wants to be a support for him. You can always make an excuse and say he got his dates mixed up and needs to be there for his friend to look out for him, to look after him. These things do happen and at the moment hes on boyfriend level, and sounds a fairly newish relationship. Just dont get mad at what ever decision he makes. If you put your foot down and say hes got to come to the wedding, then he turns up as he feels obliged to, and probably be in a mood during the evening. So see what he says, but also make sure hes aware times a ticking and people need to know.