I would say talk it through with him, as it's a shock to both of you.
If this is a bit of a miracle, just think that fate has given you the opportunity you never thought you'd have.
Your DC won't be traumatised, as much as any other child their age who gets a sibling. If anything they're at a great age to play a part in it all as an older sibling.
Yes it could be tough, but at the same time you've done it before, some men are hands on, some aren't. If you have health issues you could look into Direct Payments to help with care anyway, I've known a few women who got given extra hours to support them with a new baby.
As someone who's needed fertility treatment, I would count this as a blessing, as you don't know if it's going to happen again. If DP wants children, ok it's fairly early in your relationship, but there's never really a good time to start TTC. Yes ok, maybe when you're at the beginning you would wait a bit longer, I also think everything happens for a reason.
Being pregnant doesn't mean he has to move in and play a big part in it all. It doesn't mean you have to get married.
Would something you could say be I'm not expecting full commitment, that would be nice as we've got a connection, but we could see how things go & if they don't work out romantically I'd hope we could still be friends, you play as big or little a part in this as you want. Slowly introduce DC to them if they're happy with that.
I guess I look like a total optimist, I just think given your circumstances, how would you feel if you never got the chance again?