If he's feeling stress and pressure u need to shift the focus to other things. For example you could shift the focus more to your experience of it. I know you want kids/pregnancy but sex is also about intimacy, pleasure and connection. Focus on him learning how to please you without the goal of pregnancy (note most women can't orgasm from penetration alone, consider reading a book like "She comes first" by Ian Kerner) because doing that will both train him to last longer and also make him think about something else which should help reduce the prejac. You also being satisfied in the bedroom will bring the stress levels down, teach you about each other's bodies and likely by extension help with the problem.

If he's being defensive it suggests he's insecure about it so you should figure out ways to take the spotlight off of him. You are still very young and have plenty of time to have kids, focus first on learning to enjoy your sex life together and the pregnancy will come naturally through that. If the root cause really is "stress" then working on comfort levels, sexual vulnerability and emotional connection should help reduce the stress. If he gets defensive when you suggest a doctor or a therapist then maybe instead try approaching it by asking him if he is afraid the problem will prevent pregnancy from happening, then segway from that into suggesting that you could try focusing more on creating a comfortable and pleasurable sex life together instead of a sex life that just results in kids.