Cheating boyfriend 💔

i need some advice on what to do. I have been with my boyfriend for 4 1/2 years now. We have been through a lot and have overcome it all, but there’s a problem that I can’t take anymore and I need to know it’s not just me. I bought him a new phone for his birthday(his old phone battery charged but dies quick) and ever since his old one has been in the car. He works mon to fri and takes the car everyday. The other day I told him I was going to my moms for family dinner. He took the phone out of the car and brought it inside(he wasn’t sure if he was going with me yet). I realized it was missing when we got in the car but waited to ask. He has a routine every night where he smokes in our car so I waited to see if he would take the phone back at night, of course he did. I asked him why, He ended up telling me he was trying to get it to charge to get his sisters number and see if she texted him on his birthday, but it didn’t charge he couldn’t get it to turn on. Fast forward to today, I’m going to have our car for the day. So, I automatically checked his hiding spot in the house(he knows I know about it) and there was the phone. I decided to see if he was telling the truth about the phone so I tried charging it, it charged and turned on right away, but I also noticed he changed the passcode from the 2 I know he uses. This might seem childish or overbearing but every time I have bought him a phone, he has done this. I have let it go every time but I can’t handle it anymore. I thought our relationship had gotten better after a serious heartfelt conversation but now I’m second guessing everything. I rely on this man fully right now and it kills me inside cause I have a feeling their might be something. We are currently saving for a house with his income. I’m 9 months pregnant with our 3rd child and a stay at home mom. I don’t know what to do and need some advice.