If it were me, and I have been in a similar situation, I would respond with,
"This is your baby, if you don’t want to believe that it’s fine, but you will be the one paying for the DNA test because it’s not me with the doubts. I am having this baby with or without you allthough I’d much prefer that we did this together. You will be paying child support wether you want to or not, it’s not an optional thing, it’s the law. If you choose not to support me then it will be you who misses out on a wonderful experience of parenthood and every day you miss is one you can never recover, from now onwards. Think about it, and get back to me when you know what you are doing because I’m busy nuturing our unborn child and quite frankly I don’t need the stress ."
Then leave him a few days, there’s not alot else you can do. It is also worth saying that this will be s very stressful time of change for both of you, but this doesn’t give him the right to tkae his worries out on you, tell him to find someone else to whinge at!