Lying

So my boyfriend has a daughter from a previous relationship. I’m trying to ease into the situation as best as I can. We just moved in with each other, so we are starting to have her every other weekend. Today is her birthday and was supposed to be our weekend, but obviously Mom wanted her..I suggested we have her tomorrow to celebrate her birthday, and he said no.

A few days ago, he said he “might” have to work Sunday, (which he never does), so I am now kind of assuming it’s because he wants to go to BM’s house to celebrate her birthday..

And he brought up that he “needs” not “wants” to buy her a dollhouse. So I asked if that’s what they asked him to get her. He said no. He was sleeping and his phone went off, so I checked it. It was BM that was texting him about what to get the child...

Why is he constantly lying about talking to them?