So it’s best for him to not rock the boat as at the minute he has me, he has his 3 boys, so it’s not in his interest to bring it up.
But from his perspective there's nothing to bring up - you're the one changing the terms of the relationship.
Have you communicated to him how deeply you want a child? Because even on this thread your initial words were ambivalent, and it only became apparent how much you wanted a family after a few questions from posters. Of course, you had quite valid reasons for this. But from his perspective he's been open from the get-go that more children are not an option for him. He knew you were ambivalent, probably leaning in the other direction, but your actions in the past two years have indicated that you're fine with getting into a childless relationship.
And yes, there are anecdotes about men willing to reverse vasectomies - that might be the case. But in most cases, people who undergo surgery to end their fertility don't do so lightly! It's an act of finality and should be respected as such. Have you asked him why he took that step rather than other preventative methods?
So you're going to have to prepare yourself to hear "no" from him - and he might be the one feeling betrayed, as your actions in this relationship so far have indicated to him that you're content with a childless future with him. And if your desire for a family outweighs that, you'll have to leave him.
Please don't get taken in by the lunatic idea that reversing his vasectomy is a test of his love or commitment to you. Would you appreciate being told that giving up your dream of a family is a test of your love and commitment to him? It's a sad situation if you're not on the same page, but it's just incompatibility, not insufficient love.
Of course he could well be one of the few cases happy to reverse a surgical procedure originally intended to be permanent - I hope that's the case for you. But I don't expect it.