I want him to have a fulfilling life and I believe having kids will be fulfilling to him.

You didn't raise this guy. You don't get to tell him how to have a fulfilling life. You don't get input into those decisions and frankly it's pretty fucking nervy of you to think you do. I say this as somebody who has never wanted kids and doesn't much care for them. You need to know your place, and it ain't telling him he shouldn't have kids now. Just tell him your relationship has an expiration date but that you'd like to enjoy adventures with him now. And let him decide if that's still something he'd like to do. But you do NOT get to try to decide his timeline for having a family. It's weird that you think you get input into this. You need to learn appropriate boundaries.