First of all am very sorry to hear this, you must still be in shock, don't be embarrassed or ashamed to confide in a trusted friend or your family.
Right now you must feel like your world has caved in, it is probably for the best at present that there is some space between you so you can take it all in. Try and stay in touch for your son's sake, however his mum has changed she shouldn't attempt to distance you from your son. If she has been planning this for a while she may well have prepared the ground with her family beforehand - exiting the marriage is not something she'd have done lightly no matter how selfish she seems to be.
Relationships do fail and it often isn't so much one person 'failing' as both people unable to get along together. The mature grown up way to go about calling it a day is not to start sleeping with someone else (in her case the boss) before discussing it. She's had months to allow herself to break rules and do as she pleases, perhaps make more permanent plans, you are still playing catch up.
Too late now so best thing is to get outside unbiased help, talk things over through mediation, it isn't point scoring it should help get things on a civil footing so you have access to your son and can make arrangements to ensure his well-being.
On a practical level keep eating, don't look for comfort in alcohol, please reach out to someone close you can unburden yourself to, this is an unexpected end to an 8 year relationship but just take one day at a time.