This is not anyone's fault.
Your conversation with him should start like that. We all have ideals and life plans but these change with experience. I never wanted to have kids and neither did my dh, now we have 2, hey ho.
I think you need to be very very honest and frank with him and be prepared for some very difficult conversations with recriminations.
Yes you did originally say that you would like 2 and he has built his hopes up from there. So you need to tell him why you have changed your mind and how sorry you are that it's happened, but that if you are forced to have another you may end up resenting not just him but that second child.
You also do need to consider the possibility that you may not be able to conceive another or that the baby might have complications or something wrong.
You'll need to listen to him as he's going to be gutted and will try anything to get you to change your mind.
I suggest Relate at this point otherwise the atmosphere could get nasty. At least a counsellor will help to keep you two on track.
What is more important to you both now? Your existing son and your marriage? Or another baby? Either way your marriage might come under so much strain that it collapses anyway. Is it fair to bring another child into that situation?
I do not envy you one bit. Tread carefully, try not to let yourself get dragged into slanging matches and do stress that this is not about the wellbeing of your ds now, this is about the wellbeing of your relationship. The goalposts have been moved, yes that this a bit shit but that's life for you and it's not your fault or his. You are not being awkward for the sake of it and no you won't change your mind. So you need to decide, both of you, how to go forward from here.