Allalone it looks very much like this is a spur of the moment thing and he's possibly hoping you'll beg him not to go.
Also his rushing off from the dc and being out drinking with his friends may be because he finds it hard saying goodbye and then goes to drown his sorrows.
And yes it is easy to feel resentful of his lack of responsibility and freedom, but look what you get instead. He may well feel equally resentful that you get to be with your dc and he is sidelined to a bit part player on the lines of a favourite uncle or grandparent who gets to spoil and treat the dc.
I'm sure you both see the situation very differently from each other and it's hard to empathise with the other when their view is probably quite different to yours.
Have you told him you now know the dc put him up to the flowers thing? If not maybe it's best to get that out in the open at this stage.
He may well have thought that the request from them was at something you said and he may have had some hope that possibly there was a chance of a reconcilliation.
And any wounded animal will lash out.
Perhaps tell him you appreciate the sentiment and, in hind sight and knowing the dc had put him up to it, you would perhaps have reated differently. But does he, in hindsight, not see that it was inappropriate...
I think you both need to talk here. Never easy when people are hurting this much.
A break away may in fact be what he needs to clarify in his mind what he wants moving forward. But he needs to be the one to tell thw dc and work out how to maintain contact while he is away. You could offer suggestions such as skype, postcards etc. But ultimately it is up to him
If he really is going to go back permanently run away he may be suffering some deep emotion pain and hurt in order to think that is a better option rather than stay and parent his dc.
Some people simply can't cope with relationship breakdown and not seeing their dc as much as they want. Do you think that is possible here?