I never had this issue, but... it could be the extra discharge in pregnancy which causes you to be more "wet" and can make it feel not as tight.
&& a side note; the fact he even said this to you is rude, heck even I'm offended! Lol
Also my sister, while she was pregnant, her ex always said she was cheating "oh it's not like it use to be" Turns out he was cheating. This is probably not the fact with your mate, but if he has been with someone else, whom is well....tighter than you, then has sex with you again of course he can feel the difference.
Its your muscles relaxing.
Your vaginal muscle will feel tight, but the more turned on (aka more relaxed) you are, the "looser" you will be.
Try doing some keagals during sex. That will give him a tighter feeling, and help you out too.
Also, if he ever makes you feel bad about it, look at him and say he feels smaller than usual. ;P
LMAOO THE OTHER COMMENTS. YOU COULD HAVE TO MUCH DISCHARGE AND IT JUST FEEL LOOSE. MY SISTER SAID SHE FELT THE SAME WAY IN HER PREGNANCY. SHE USE TO TAKE A HOT BATH EVERYDAY AND DO KEGALS IN THE BATHTUB SHE SAID IT HELPED ALOT IN A WEEK SHE NOTICED A DIFFERENCE SHE SAID SHE THOUGHT SHE DID IT TO MUCH CAUSE WHEN SHE DID HAVE SEX IT HURT CAUSE SHE FELT TO TIGHT LMAOOO I GUESS EVERYONES DIFFERNT. I NEVER HAD THE PROBLEM WELL I DONT EVEN KNOW CAUSE I WAS A SIBGLE PARENT MY PREGNANCY TILL MY DAUGHTER WAS ALMOST ONE LOL. SO I CANT GIVE TO MUCH ADVICE I CAN JUST GO OFF MY SISTERS EXPERIENCE.
It could be all in his head. You haven't even had the baby yet
Haha Good one ,Thank you for your responses ladies