Whistler, doesn't sound good does it, we've all been through 'stuff' when we were younger relationship wise and we did make it through, we all need different things and tactics to get us through these things.
When I split with my ex finance all those years ago, I had a mother who basically gave me the stiff upper lip and told me to get on with it, no good, forget him kind of thing and a dad who cuddled me and wiped every tear, but do you know what did me the most good one of my brothers took me away with his wife for a fab holiday in the sun, that is what really bucked me up and gave me the strength to carry on and be independant again.
I really think that your son needs taking away from the situation, he's being given conflicting messages from his ex, on again, ex again girlfriend, he's in a state if he is loosing weight and yes, he needs his family, but mostly he needs to be taken away from the situation to think clearly.
If I were in your situation and my son was hurting so badly I would put work on hold no matter what and take my son away or find a good friend, sibling someone for him to go with, pay for it and look after him, he needs to get away, it is what I would be doing in your situation and most people won't turn down a nice holiday.
If you have real concerns that he is reaching the edge and may be suicidle then jump in with both feet to protect him, there is no age limit on looking out for our children. :-)
All the best, blooming girl messing him up like this.