Boyfriend doesn't want to have sex now?

My boyfriend used to want to have sex all the time, but the last couple months, he hasn't wanted to at all really. He said he's still attracted to me and he still gets hard when we fool around, he just never wants to actually have sex. I'm not sure if it's because I'm pregnant or what. He knows he won't hurt the baby and he says I'm still pretty and everything, but he says he just doesn't even think about having sex anymore. I'm almost certain he's not seeing anyone else and we've talked about it. He seems concerned that something is wrong with him. Has this happened to anyone else while they were pregnant?