It's not a matter of being 'tight' and stretching you out. It's a matter of you being relaxed, and aroused enough for him to slide into you.
Maybe you two just need to spend a little more time with foreplay. Kissing, touching, fingering, him going down on you etc. It might help to relax you, and also arouse you more, and get you physically ready for him to enter you.
Hope that helps.