I think the point is shes trying to get pregnant... so in that case give him a blowjob before hand and soften him up a bit for the ride, since he has a hard time cumming. Or try something new, I dont know what your into but let him slap your ass, pull your hair idk, whatever he really is excited to do and I gaurentee a money shot. Me personally its a lil hard for me to cum, But when I get rough with a lady it comes a lot quicker lol. Seriously if you want him to cum, ask him his deepest sexual desire, let him do it to you and pray he doesnt get pissed when you have kids lol.
Ps. If you don't want to get pregnant do it with a condom on at least(trojan ecstasy feel like nothing is on almost ^^ ironically enough).