My story is very similar to yours although I have 4DS (2 with ASD) so know the awful pressure we are under. DH had a 3 month affair which culminated in him moving in with OW for six weeks.
Right now you are in shock and your emotions will swing from one extreme to the other. You need to see some action from your H now and I would suggest he takes some emergency leave from work (his boss already knows so it will be no surprise) and he comes and looks after your DS so you can get some space and peace - he's had a year of escaping from it
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Make no life changing decisions as this time (3 months on is generall thought of as best when things have settled). You may find Relate very useful but give it a few weeks as you are too traumatised atm and with hindsight, me and DH went too soon.
Try and get hold of Not Just Friends by Shirley Glass and check out this site - I found it very useful in the early days (helped me through some very dark nights). I also phoned the Samaritans at 2.00am one morning to talk to someone as I was feeling that back.
One day at a time, and be selfish now, think about what you want and how you want your life to change - we give up so much of ourselves to our families and we lose ourselves in the process
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This article may help you too, get some insight into things.
Take care and be kind to yourself - you may find you won't have a weight problem after all this (most of us seem to lose copious amount so weight following discovery - about the only silver lining). BTW we are 2.5+ years on now and stronger than ever 