Hi, M.!
I'd have to say I think that his reaction is probably pretty normal. I mean, that is a lot for an 8 year old to compute. He's probably just trying to process the idea and figure out how that'll affect him, ya know? I'd suggest that you ask him if he has any questions and let him feel safe in expressing his feelings about it all. It doesn't mean that he gets to make decisions about your life, but knowing how he feels and helping him work through those feelings would be a great thing!
We have a yours-mine & ours family and it's worked out great. We have very open communication with our kiddos who are 8 & 9 (and from our first marriages.) Even after we've been married for over 4 years, questions still come up sometimes and feelings need to be talked about.
Best of luck with your upcoming marriage!