You have 4 weeks before your departure and anything can happen in those 4 weeks. For now, I wouldn't worry about contacting him, i.e., wait and see what happens and if you really feel the need to say goodbye, then do so the morning you leave.
At this time, I wouldn't tell him what your plans are to return. Has he been open w/you about his comings and goings? Your leaving most likely will not have any affect on him right now because he's in the selfish "me, me, me" mode.
Your paths will cross again and again because of your children. However, it's going to take some time for him to settle down into his little fantasy world. As for him being happy, he may appear to be happy right now because his new found freedom is light a shiny new toy and once the shine wears off and the newness of his situation becomes a routine, things will change. Many of them will appear to be happy around you and you can take this one to the bank...they won't tell you they made a mistake or they are unhappy. If you are one of the lucky ones, at the end of his journey, when he wakes up, he may share w/you about his feelings and admit that he wasn't happy...but that's a long way down the path.
For now, focus on you, your trip and your S18. Leave the man/child twirling in the wind.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
