I can (could?) completely relate to those feelings. Never think they're not valid, because they are! You're right -- things ARE about to change... and honestly, the relationship you have with your son *won't* be the same later... But just because it will be different doesn't mean it's bad!! ... I think a mother experiences feelings like this especially with the birth of a second child (I know I did... and I felt terrible about so much for it).
AND, I can relate to getting a bit... hmm... nervous? .. about the prospect of your relatively easy life with a five-year-old getting completely overhauled by a tiny baby who doesn't need much, but does demand a TON of TIME! My girls were (haha.. and still are) 8, 6, and 4 when dd4 was born. They're pretty independent and quite... easy! I could just send them all out to play in the yard so I could get stuff done! .. Now? Holy heck, I tried to figure it out one day (but can't recall.. such is my brain these days), but I spend a TON of hours just SITTING -- because nursing takes a lot of time! Hours and hours per day!
BUT... I still think it's worth it

I'm glad you're able to realize and share these feelings now.. it's best to work through them and process them before birth.. because something like this could stall things I think (I ran into some huge issues/stalls during dd4s birth). You *can* take time to 'grieve' for the relationship that won't ever be the same with your ds... it's okay! And healthy even! ... I think getting past that or working through it will make life with your new babe even more special.
(or.. that's my wordy hey-I-have-two-hands-because-baby-is-sleeping-on-my-lap response)
[[hugs]]