Caught In Between Him and His Mama

I have a beautiful relationship with my man of a 1 year and a half. I am very much in love with him and everything is going great. We have a son on the way, that's due on new year's day. See, the thing is, it's his mama. See, she's really sweet, caring, and has never shown me a hostile side ever. But, it's like we're both competing for my man's time. I almost feels like sometimes she's an ex instead of his mom.She always wants to know where he is, she interrupts lunch dates, and she has him run errands that put off our plans. It's really starting to get annoying. I mean, he loves his mom, because his father wasn't in his life and she raised him alone. But, god, sometimes i wonder what's really up because they're too close, almost to the point where i feel like im a third wheel and VERY uncomfortable. Also, his mother doesn't have a man, and hasn't had one in a while, and ever since she hasn't had one, she's been VERY much involved in our relationship. please help!?!?!