Why won't my boyfriend have sex with me

I'm 21 and my boyfriend is 24 we've been together a few years we live together and TTC for our second baby, we've never had a problem when it comes to sex and he's normally the one that comes onto me but for the past few weeks he's not been interested at all and everytime I try he pushes me away and says he's either not in mood or is tiered, what should I do as I feel he's just giving up ?