Baby mama drama

Hey ladies so I’m Ive been dating my boyfriend for a year now. And he has a son Sammy and I of course see him as my own he just turned 3 but we didn’t get to see him for his birthday which was the 12th of this month. We haven’t seen him even before that’s it’s Almost been a month of not seeing our son. We have called his biological mother multiple times she never picked we have texted she usually leaves us on read or never sees it. And the very rare time she has responded she says she’s out of town and didn’t come back till “last night” we keep asking for him she’s never responds. We haven’t seen him or photos of him till a week Before his birthday. she just replied my boyfriend today saying that she might not give us Sammy because he has a dentist appointment on our day “Monday” we can take him tho. I just don’t know what to do. My boyfriend is frustrated and so am I. We try to stay calm and respectful towards her but how do I support him/ confront him? And how do we get to see or baby?