How do i make my husband want sex again?

Ok me and my husband have been togther for almost 2 yrs. we have a 8 month old son and we have only been married for 7 months. At first he wanted sex all the time i mean i could look at him and he would be ready to go. But seems like now he doesnt want it that much (like 2-3 times a month). i ask him what is wrong and he says nothing, but i wanna know...but i dont want to sound like i am nagging. He is 30. I am 27. I think it might have something to do with the fact that he saw our son be born. He says he does look at me different. meaning that he used to see me as his sex kitten now he sees me as a mommy. Or if he says something he says he has put on a few pounds and he just isnt in the mood....i know he is NOT cheating. I just wish i knew what to do..please help!!!