ITH- don't just be polite and casual. Be asleep! Don't even speak to him when he gets in.
I don't think he's gone out drinking to show you the M means nothing. He might just need a drink/to drown his sorrows after a hard day today. No ASS-U-ME-ing, remember!
I don't think this is the end for you. You can turn things around but you'll need patience. Back off him now and let the duct settle. No R talks, no talks of any kind other than polite interaction which you end first. Be less available on IM etc. He shouldn't be able to contact you whenever he wants to, and you being less available should pique his curiosity.
Finally, re-read the 6 stages of MLC. Your H may have been in denial/anger before and may be moving to some more anger/reply type behaviours. I'm wary of putting labels and stages on things, but try to think of this as a marathon. At the moment you're going up a hill, but eventually you'll get to the other side and going down will be easier. It's just not straight up and down- there are undulations to the territory. (sorry)
