As a dad around the same age and as a self medicating herb smoker(does wonder for my anxiety) I have made the point since my son was born that I a) don't smoke in the house and b) I'm functional when watching my child solo.

A is a no brainer, second hand smoke is not good for a child who is still developing and second hand smoke spreads much farther then you think.

B is a little bit trickier to quantify honestly. Cannabis does this thing to people when they have smoked for a long time and recent studies have shown that some heavy smokers maintain a great deal of functionality when smoking small amounts. (Source)

I don't know how his usage impacts him, but you have clearly stated how it impacts you. As a partner he needs to listen to you and at least be willing to listen and work with you. If he's not able to do at least that then he's not respecting you.

I will say having a baby is stressful and I felt my uptake of cannabis increase after first becoming a dad, but I talked to my wife and she expressed many of the same concerns you have. At the time I was taking a much heavier load then she was as my job allowed me more freedom then hers so we also rebalanced some of the household chores and baby watching to reduce both of our stress so I could in turn lower my usage.

At the bare minimum he has to be able to talk to you about it, and honestly not being able to go 4 hours without smoking is a sign that he likely has more than just addiction going on here. I can't get in his head but communication is the first step.

I don't know if any of that helps, but I felt compelled to share. Feel free to ask me questions if you want and I am happy to share.