I realise this is a sensitive topic and some people can get upset with detail, so happy to move to PM in confidence if that helps.
Have you tried tensing while he is inside you - in effect you doing a kegel exercise? That can really help.
It's always tighter going in, that's normal. The sensitive part of the penis is the head. Try not going as deep?
Do you use lube? Is he smaller in the girth/length department? Circumcised?
Maybe google coital alignment technique - where the penis is rubbed against the front wall of the vagina and see if that provides more contact. Get the Karma Sutra out and see if any positions better than others.
The more turned on the woman becomes the longer, wider and wetter the vagina becomes. Against usual advice, maybe try less foreplay. Avoid anything up there before penetration as fingers/toys etc will stretch things out and make it looser.
Sex can be many things, no reason vaginal intercourse cannot be a part of this, even if he needs something else to finish. Worth discussing his porn and masturbation habits as if he plays with himself with a tight grip, no vagina will ever compete.
Not sure how helpful that is?