I have a 4 yrs old daughter and I can't imagine even a little bit her not being there. It's an important day for her too as she gets a new step parent and I want her to feel as much a part of the day as me.
Why can't he go to Jamaica with the two of you and come back early? This way you get the best of both worlds...he gets to attend your ceremony and then you get your alone time with your new dh afterwards. At the age of 11, he is definitely old enough to fly on his own (you can accompany him through to security and there are services for children flying alone) and his Grandma and cousins can be there to pick him up and he can still spend that time with them.
I dunno, I know this is probably not what you want to hear but coming from another mother I just can't imagine my daughter not being there to be a part of the day even though she is only 4.