I haven't read the whole thread, but have dipped in and out over the last few days.
OP: how friendly are you and your ex? is it at all likely that she owuld use this oportunity ot weaken (further) the relationship between you and your son?
my dh moved abroad for a job when his children were a similar age. he had always been hands on (EOW, one night midweek, but had also always had to travel a lot ofr his job, so these did get moved about. he did his best, within the constraints of his job, to make up the times he missed (taking into account his ex's/his children's lives too) - just saying this for background)
when the overseas job cameup, he talked it through with everyone. he woudl, of course, not be able to fulfill the EOW or midweek evenings, but the job would mean he was back in the UK every month (at least), and there was the chance of increased school holiday times (he had only been "allowed" a 2 week holiday once a year at that point - he wanted to increase htis)
so, the end result was that his ex agreed to a new contact schedule: a long (Thurs-Mon) weekend once a month when dh was back, plus extra school holiday times. (2 weeks in the summer, one at Christmas, one at Easter. half terms "ad hoc")
dh, on the basis of this agreement, went ahead and took the job (and didnt get anyhtign in writing!)
as soon as he was abroad, the ex reneged. whenever dh tried ot book in for his weekend when back, she was either uncontctable until too late, or the children were busy (doign routine things, eg team sports which dh woudl have been happy ot take them along to) and couldn't see him. the extra school holidays didn't happen either (they were always meant ot, but somehting always ballsed it up)
dh ended up in a protracted and costly court case, fighting to gain the agreement he had made before he went, with his ex throwing it all back in his face.
Now I know you are not the one moving, OP - but is it at all possible that your ex woudl end up making things difficult for access if she did go abroad with your son?
there are lots of posters here saying "you cannot weaken the relationship if it is strong in the first place". well, I am afraid oyu can. dh's children were never aware that their mother had prevented him seeing them - she told them he was the busy one, who hadn't arranged contact "never mind, maybe next time" etc.
it can all go so wrong, especially if one side ends up with too much "power" and "control" in organising the contact.