Is he hiding something?

okay so long story short I have a son from a previous relationship but my so has been raising him since he was 2 months old his dad left ...currently my so and I are ttc for a while now and I said maybe we can get him checked to see if he can have kids and he says " I know I can have kids" and I simply reply how do you know if you don't have any kids of ur own or never tired with anyone else and he just replies "Because i know I can" I feel like I'm missing something or he's hiding something.