@AllOfThemWitches yes, this is exactly why she needs the baby's dad, who she's updated and said she's known for years, to be around to help her. As I said in my previous posts most people don't have a network of friends and family willing to drop their own lives and provide round the clock support to a mum and baby- that's the baby's dads job.
Op i do think it's sensible as PP have said to take evey step you can to safeguard your child against new bf possibly being a paedo or abuser, but equally I don't think if you wait another 6m-1y you'll be any the wiser about this - you've known him for a long time and you haven't heard any horror stories. And also, if he's there a lot helping with baby but not staying there then he'd also have access to both your kids.
I think the best situation is to be there as though living in but to have somewhere to go (his own place or parents etc) if you argue or want space or want him to leave/break up with him. Or if worst case scenario he is not nice to your big boy or you.
Also, I think PPs have been very negative assuming your 5 year old is going to hate having a baby around and this will turn his whole world upside down- children often love their baby siblings, yes it will be a change but it could be a lovely addition for him. Just needs very careful managing so he doesn't feel confused or pushed out.
OP please also ignore the particularly really nasty PP who seems determined to make you feel as small and stupid as possible which she judges you from up at her pulpit. Unplanned pregnancies can happen to anyone who ever has sex and this baby sounds like a wonderful blessing into your little family. There are lots of stories here about all this working out, just be careful and have a 'get out' plan set up.
I would also chat to someone pastoral at your sons school about what's happening- if your son has special needs they can advise you on books or things like social stories to prepare your big boy for the changes ahead. There might be a book you could even by about blended families or step dads or half baby siblings etc.