AIW for being upset that my boyfriend is going on vacation with ex-wife and kid?

35 female here. 38M boyfriend.

Am I wrong for being upset? I have been with this guy for a year now and he just told me he's going on vacation with his ex-wife and their son (who's 8). They're going on a 4/5 day trip. They've been divorced for a few years now and usually vacation once a year or so together. I haven't met his ex or kid yet, but will soon.
Should I talk to him about this? Why are they going on vacation together? Do you think he's using me as a back up option but wants his ex back?