I'm gonna start off by saying; I'm not gonna go into the various accusations om this thread and only stick to what you have said.
I mean absolutely no disrespect and truely hope you will read this with an open heart, because I think some of the thread is going off om a tangent and missing the actual point.
This is not to flame you, but I hope it can open your eyes to what I feel you should focus on
You said...:
"My son did nothing wrong. I cannot get through to my husband. He reiterates that I overreacted and need to let it go. He was a total arse though."
"Again, this is detracting from the issue I have, which is...he has not bonded well with my son. He has with my daughter. He is sometimes unkind, or hard on my son. I don't understand why."
"I think my daughter is some sort of compensation for the loss of HIS daughter. He has his son, so doesn't need that 'role' fulfilled. When he chooses to engage (for example, with my daughter, his son, his nieces and nephews etc...) he is a really committed and loyal person and is adored by them all.
It is just my boy."
With all due respect - it does not matter why. It does not matter that you want to understand (although I understand your desperate need to have some sort of reason that could excuse the behaviour, since you love him).
What matters is the treatment of your son.
You absolutely know there is a problem with your husband's treatment of your son. He doesn't. He thinks you are overreacting and doesn't think anything is wrong with the way he is treating your son. That is ALL you need to known.
You cannot make him change by you trying to understand the reason behind his actions, if he is not willing to see it himself.
Fair enough if there was a genuine reason behind the way he acts and when confronted by it he was appaled at his own actions and willing to change. However he isn't. He thinks you are overreacting and he hasn't done anything wrong.
You are your son's protector. You have brought in a man into his life, who is not treating him right. You are the one to correct this mistake, not some man who can't even see there is a problem.
It is not easy and particularly heartbreaking for you, since you already have a divorce behind you and perhaps this man brings you a lot of joy you never thought you could have again. Truely, I get it must be hard.
But if this man is not willing or able to see his mistreatment of your son, you need to take steps to protect him. Merely sticking up for him to your husband is not protection. He is still being subjected to it.
"I posted BECAUSE of my son. I literally don't know what to do other than issue an ultimatum"
Glad to hear you are having a talk tonight. Please make sure that you listen to what he says. His reaction and possible remorse or not is what you should be looking for. It will give you the answer you are looking for.
Good luck.