Not sure if you are having a planned cs or not but any chance of reminding him that dates are 'very vague' and it could be two or three weeks afterwards that new baby is actually born... And that if you end up having a CSection then you could well be in hospital for at least a week plus recuperation time at home. Harder toll on your body second time around so more recovery time plus extra stresses of looking after ds1.
OK so the above is probably a fairly generous timeline but I am guessing that he doesn't have children of his own from the way you write about him so he isn't likely to be up to date on the fact that they aren't likely to let you go past 2 weeks over or that they will do their best to kick you out a couple of days after a csection or who knows if it is easier or harder second time around... but it might help to persuade him to plan on coming later when the baby is actually born and settled rather than being available to turn up as soon as the baby arrives.
THe other tack to try is to say that, knowing how hard it was first time around and reckoning that it will be harder this time around, you want to try to spread your visitors out so that you get support over the first few weeks rather than trying to be a superhost to everyone straight away. So your dh has got a couple of weeks paternity leave, and then your mum/mil/whoever is going to come but you would love it if they came after that.
On the other hand, maybe you want short sharp burst of everyone together so that you can hide and leave them to amuse each other?
If you are happy for them to look after DS1, you can also start mentioning how fantastic it will be for ds1 to have uncle gsheep there to play with him while you have to spend hours locked away mumsnetting napping watching tv looking after and bonding with the new baby as you are really worried how you will cope looking after him when you know how much time new babies can tie you up for. and how much ds1 is looking forward to playing with uncle and uncle's gf, and comparing their cooking / story reading / football / nappy changing / etc etc to mummy's and so on... also might make them realise that maybe he has a different agenda for the visit than you do!
good luck having your baby and hope that it all goes ok afterwards...