Bf fb message and snap other girl

My bf and I have been together for almost 2 years, our son is 10 months old. He has a work friend that is a girl and they have a past of being friends.

She messages him all the time and he answers back innocently, but then he started deleting the conversations, now they are snapping and she is a best friend on his snap ,and all he says when I ask what they were talking about was idk.

I am not a very trusting person but I have been trying to be cool about this because I know he is allowed to have friends that are girls and he is allowed to talk to them, he has lied in the past when it comes to girls, so my only request was that no messages were deleted so I knew for sure he has nothing to hide.... now they are snapping, and fb messages are being deleted.

Anyways my question is am I being irrational? Should I ask him to stop talking to her altogether?

I just don't want to be controlling

Thanks in advance ☺