Hubby can’t finish

Hey girls, my husband and me have a great sex life, but sometimes he has difficulty getting off... sometimes it takes 5 minutes and then some times he literally can go for hours but not cum, he swears it’s not me but I can’t help but to feel like he’s enjoying it.. he’s not old (he’s 21) and he has no issue getting it up and keeping it up, it’s just him not finishing 😕 we’re ttc baby #1 but this last cycle he couldn’t finish on my most fertile days, I thought it’s maybe because he’s having doubts but he says he 100% wants a baby

any tips? Advice?