Full size bed transition

So with second baby coming in a few months and son turning 3 next week…we made his present his “big boy room” which includes a full size bed. He transitioned to a toddler bed shortly after he turned 2 and did very well. I wasn’t anticipating issues with this transition but man last night was rough. He was awake for 3 hours screaming and crying about not liking his bed :( any words of advice for anyone who’s done this yet? I’m hoping it’s just a few day adjustment….