First, welcome to the MH pages. People here try to help each other.
Oh dear, this is not good. I suspect you have a number of issues rolling round in your head. It may help to unpick them appart:-
You have a history off issues from having "difficult" parents.
You are scared to rock the boat.
New mum with all the normal mum issues... is he sleeping etc
Major focus on son as the light of your life..
Partner works long hours and you want him to do more to care for son.
Partner says you are no longer fun.
You feel detached when you think about partner.
You wonder about leaving him, but you do not have anyone else in view.
Have .i missed anything?
At this point I will try and tread carefully.. Please forgive if I get things a bit wrong.
It seems that when you met partner, you were fun.. (partners description). Now you have become a mother and no longer prepared to rock the boat. Partner is missing his fun girlfriend. He is also tired from long hours. And when he gets home you need his help with son but he wants/needs relief from work pressures. This is going to cause problems in the relationship.
Into this you have all the usual issues of being a mother; tired, hormone changes, major focus on child as first call on your emotion, tiredness etc. You have changed. That is not said to imply criticisism. You have to change from a carefree teenager into a mother in order to successfully raise your child.
As when all things change life becomes uncomfortable and uncertain. That also leads to discomfort and unhappyness. You are experiencing the effects of major life changing events on top of a history of "issues".
I will not try to suggest a course of action as only you can decide what you should do. By way of observations, you have not said anything about abusive behaviour. That is good as it leaves you the basic choices, repair the relationship or break it. I would not suggest repair if he is physically or mentally abusive. Whichever you take, there will be a rough patch ahead. But you have the strength of a woman.
Please keep posting. Writing things down often helps..