Christina,
It is SO important that your child know and get to experience both parts of his heritage. Both of my children are of mixed race, so I know how difficult it can be.
I think your husband is just being protective over your child. Three weeks is a long time to go without them especially being only 2 1/2. Plus, it's so far away, it's not like its a couple of hundred miles. However, you might want to sit down with your husband and have a serious discussion about this. It's imperative that your son know as much about himself as possible, and that includes interacting with your family too.
Maybe instead of your son staying so long, he could go back with your cousin and then you go back to get him a week later? Or make it a family trip, and everyone go. I also agree with what one reader responded, that your son be the deciding factor. ALL children are different, and he may take to your cousin very well and may be excited about going. I would never keep my child from exploring just becuase I would miss them. It would have to envlove a danger to their wellbeing, and I don't believe that you would do that.
I really hope this helps. And NEVER give up trying, if your husband loves you, he will come around.
If you need to talk, feel free to email me at ____@____.com
Regards,
T. C.