Is there any way to roll him on his side, and away from the top of the stairs ?
I don't know what your DH is lke when he has had a lot to drink (ie does he get violent, or just silly, or would he start shouting if he wakes) but
a) if he is sick and lying on his back, he could choke, hence suggest trying to leave him on his side
b) I suggest rolling him away from stairs as he might wake and try to stagger to loo, but fall down stairs
c) if you are at risk if you wake him do as little as possible, especially if he will wake your DS1, and frighten him, let alone DC2.
Good you have had chance to sleep, and hope DS1 can be noisy early on and give Daddy a rude awakening (I am imagining the Specsavers advert where a tearaway child rampages around on sofa, and his Dad finds glasses bent and with one lens smashed)...
Such a 'wake up Daddy' event could be a good reminder not to do this again (though I don't suppose your youngster is likely to run around quite so boisterously, and reaction from Daddy needs to be predictable and not a slap!
Of course, you could provide Daddy with the smell of bacon and egg around 8am assuming he is awake enough to notice, just to get him back into the real 'family' world, and then after DS1 is awake and ready to play, best take DC2 and yourself back to bed as you 'had a disturbed night' (!!)