hi,
Perhaps i can give a perspective from the other side of the fence, as a similarly "reluctant dad".
Prior to marrying my DW, we had "the chat" about kids - i wasn't particularly keen (to say the least), however DW basically said that if I was absolutely sure I didn't want kids, then our relationship was over, so that she could find a man who did... Faced with that ultimatum, I agreed. (and more or less tried not to think about it)
What helped me get my head around it was trying to examine why I wasn't keen on kids, and what I could do about it. Reading a lot of books was (mostly) helpful for this. In the end these were the things I was worried about:
Financial responsibility - aka growing up (a real, and underestimated issue for a lot of men, but we were fortunate enough to have anough cash that it probably wasn't going to kill us).
Baby yuck - poo, vomit, nappy changes etc. (really not a big deal in the end run)
Change of lifestyle (not a huge one again - me and DW were/are homebodies much of the time - though I did have a huge lads holiday prior to the birth to get a last big blowout in)
Change of relationship with DW (this was the biggie. And as yesireallycan says, there is no way round it - for anywhere between 1 month and a couple of years your relationship will be treading water. But the good news, is that unlike most other things, this is something you can do something about)
The three best books for help I found were:
babyshock (from relate)
life after birth (kate figes)
baby proofing your marriage
The worst one were: pretty much any fatherhood book. (Oh no, we couldn't possibly talk seriously about how kids affect our relationship/sex life, etc, no, n, we're men, so we'll joke about it. feh)
So the advise I would give to your DH would be:
(a) try and examine why you don't want kids - some of the reasons can be resolved, some just have to be lived with.
(b) get involved. right from the start. If you are just a bystander, you will just feel (more) excluded. If you are involved, it will become more of a "joint project"
(c) be prepared to feel utterly excluded and a fifth wheel after the birth. these are pretty normal feelings.
(d) for gods sake don't do anything stupid because of those feelings.
(e) talk, talk, talk. Keeping the lines of communication open is critical, esp in the case where one partner is reluctant.
(f) talk privately to male friends with kids. they will likely tell you how much the change has made them happier.
(g) go to the antenatal class. go to the antenatal class. go to the antenatal class.
(h) decide (the two of you) what "little things" are important to the two of you as a couple. Do your utmost to keep those up - even in the darkest sleep deprived days.
(i) as a man, for a lot of reason the bad bits (and there are plenty) of kids are easier to visualise that the "good bits". and tbh, if you can't visualise them now, then you will just have to take peoples word for it!
(j) you wont really "get it wrong" as far as looking after babies go. really. and tbh the whole process just happens (admittedly with a lot of work).
(k) More seriously, after you have examined your reasons why you dont want kids, be honest with yourself. If you truly don't want them (and it's ok not to), you owe it to all concerned to say so. Yes that will mean you and your DW choosing between kids and your marriage. Which sucks. but it's better than having a child which is unwanted.
Summarising: If your DH is worried about your relationship, the bad news is he should be - it will change completely, and not always for the better. But the good news is that there is many things you can do about it. And in time, you will likely see that it has changed your relationship massively for the better.
(apologies for long rambling post)