Moms with other children---Help!

I don't know what to do anymore. My 3 almost 4 year old will NOT sleep through the night. Bedtime us becoming a chore and is taking too long. We recently changed him out of a toddler bed (need crib for baby #2) into a twin bed. He slept great in his toddler bed with the rail. This bed has no rail and for some reason he can't sleep a full night?! He always comes into our bed around 2,3, 4am. And we are both exhausted and me being 8 months pregnant just let him sleep in our bed for the rest of the night. I know this is not healthy and he needs to be in his own bed---but it's getting bad. I need help/advice on what we can do?! Hubby and I are so getting so irritable and angry and just need SLEEP!! Thanks ladies 🙏🏻