Babies last name

So here’s some back story ... I’m going to be a very young first time mom ... I’ll be 18 10 days before my baby’s due date... I’ve been living and in a relationship with the baby’s dad for over 2 years now I really found out he wasn’t being loyal (again) getting girls numbers texting them behind my back as if he’s single and flirting with them (he said nothing ever happened and it didn’t last long but it’s the fact that there was multiple over a pretty long period of time so I don’t believe the oh it was a mistake b.s. plus the fact he denied it when confronted about it) He’s been very supportive paying all of our bills having me stay home to rest and focus on school , buying lots of baby stuff since the day he found out, he’s gone to every single doctor appointment with me. We originally got engaged after we found out but since I made him take the ring back I was uncomfortable wearing $3000 on my finger when we have more important things we can spend them at on (plus I’m not a jewelry person and didn’t want a ring in the first place) but now as the days getting closer he’s made comments saying if our son doesn’t have his last name he won’t be in his or my life, I’m questioning only using my last name because he hasn’t been honest and loyal to me and he claims one day I’ll have his last name but there’s definitely no telling if that will ever happen basset his recent choices. I feel since he’s against him having my last name he’s going to use that as a way to take our son or like he’s planning something idk I’ve got a lot goin on and am so nervous about what choices to make that are best for my son. I love this man with all of my heart but it’s something he’s going to have to prove we can make Work, I’ve brought up the idea of having both of our last names and it’s not good enough for him. What is your advice of what I should do.