Well, I have been in a similar situation so I will try to give some advice:
There isn’t really a “good” way of doing this. If he wants to be married, and you don’t, that conflict in interest will result in some sort of emotionality whether you like it or not, I’m afraid. I think that it’s good to be prepared for that, that he can get angry, upset, stop talking to you (for longer) etc. Importantly, you must also consider his potential for physical violence and how to do this the safest way possible.
I found that starting off the conversation was best done for me by starting to pose real and confrontational questions about the relationship. (NOTE my ex was not violent one bit - please consider this first depending). Things like “are you really happy?” “Where are going with this relationship?” For me, this was hard as we never spoke about anything, really, but it started a process in him to understand my thinking.
Further, cliche, but focus on you and your wishes for the future and that you need to move on. Provide real examples of why and what you aren’t happy with.
It might be that your husband will be shocked the the emotions will kick in at a later point, that happened to me. I got a lot of abusive text messages and name calling, from a man I’d never ever thought would be capable of such behaviour. He tried to stall our separation as well, as a revenge I am sure. All these things are minor in my opinion, compared to what some men do. Please keep your wits about you and be prepared.
I hope it helps and good luck 