If you are totally honest and frank with your dp then it will encourage him to lay bare his feelings with you.
Book a babysitter, take a night off and have a heart-to-heart (watch that alcohol though!). Your dh needs to consider:
How will your relationship change with another child.
What if you cannot conceive a second?
What if you have another boy? Or twins?
What if something goes wrong? Because realistically the older you are the more your risks increase for things like Downs Syndrome.
What about your mental health and risks of PND?
Would he consider adoption?
What if you get PND and cannot cope?
What if you end up resenting him and the relationship falls apart?
What if you just say no, how would he feel? What would happen?
What is the deal-breaker here? Your relationship or another baby?
Communication is the only way you can deal with this issue. And I really think that you should book a session with Relate, because they will keep you on track if your dp insists on using your ds as a tool in his argument.
No-one can predict the future, so it's crap to say that your ds needs a sibling. It's a rubbish reason to have another child. You do so because both of you want to create another life, not just because you think your ds could do with a playmate.