I have been EXACTLY where you are OP. I understand completely everything you have said.
In answer to your question there is no way to broach it other than to tell your husband the truth. If he is a decent person with any empathy at all he will understand.
I avoided my DSC for a while after my miscarriages. I was "ill" a couple of times when they stayed and made plans with my Mum at other times. My husband understood I just needed this time. It will not harm your DSS to spend some time alone with his Dad for a while, he doesn't need to know anything at all.
I felt exactly like you. It was so hard to watch my husband be everything I'd hoped to be as well. We could share this grief together when we were alone but then when DSC came he got to forget about it and be a parent in a way I could not but desperately wanted.
It's nothing to do with them individually but most people would struggle with this. Any person with half a brain cell would understand it.
Do not let anyone tell you, especially your husband, that you are "lucky" to at least have his child. I was told this and it made me furious. He absolutely must understand that you have lost your baby, his son is not a substitute and in fact is actually just a painful reminder right now.
He can still be supportive of you without forcing you into a situation that feels impossible right now. He can be supportive by understanding and ensuring you have space from his son right now which is what you need.
I'm sending so many positive thoughts and sympathies your way OP. It is absolutely gut wrenching. Especially so when your partner, the person who should be feeling this with you, has the one thing you're grieving over not having. It's absolutely awful and nearly broke me so I really do understand.
On a different note, after much heartache I do now have a 1 year old son with DH and things are much easier. Don't give up hope 💙