Do you have any reason to think he'll take it badly?
My situation is slightly different... I have 2 DSDs and have ben with their dad for many years... Certainly, neither of them can remember a time their dad and I weren't together...
Neither their mum or dad had gone on to have more children, so they were the only kids who needed to be considered IYSWIM...
We (Their dad and I) had a DS last year and are due to have a DD this year (a few weeks now).
They both took the news brilliantly and were actually at the hospital with us for the birth of DS so were first to meet him.
We told them before we told anyone else (I was about 22 weeks PG - didn't have much of a bump)
They were yr 6 and yr 9 at the time.
For us, waiting until we knew everything was OK was important (but I didn't know about the PG until relatively late on anyway) and making sure they were first to be told (apart from their mum, but that's not an issue for you! ) and making sure they felt part of the whole thing (helped choose names etc)
We told them about DS on holiday - and I'm glad it was just us, without interuptions / friends / appts etc., Meant we could talk properly. We told them about DD over dinner... The second one isn't such a big deal, is it??
Hope that's of some help.