Can I give the groom a blowjob at his bachelor party?
You probably should not. Do you think that's a good idea?
Yes, I'm going to make him cum on my face
You shouldn't be doing sexual things with someone before their wedding to another person. Unless there is consent from all parties including the bride, you should probably avoid it.
He already said that I'm his fantasy.
Maybe you should let the bride know. If the groom is emotionally cheating with you and you are considering physically cheating with him, it could hurt her a lot. I would reconsider your plan.