DS has also been totally on board and is up for a relocation. The issue is how to deal with the relationship split at the same time - I don't want him to resent me or his dad for making the choice for him, but at the same time I don't want to put him into what is essentially something that no child should have to go through and choose between his parents.
In view of this, the main problem is if your OH decides to fight for residency.
The best way may be for both of you to talk to him and present him with a choice with where to live, but making it very clear that you both know he's not choosing between parents and that both of you will be happy with his decision either way. It shouldn't have to be final, and he should know that he could change his mind at any time.
BTW, I don't criticise you AT ALL for wanting a new job, or to move, even if it means not living in the same town as your son.
You can still maintain a great relationship.
He's 13 and at an age when they start growing more distant from their parents. He should know you're there for him regardless and that he can count on your support any time.
But he doesn't need constant supervision or input from a parent.
Do you think your ex and you will be able to make a decision without a fight?
If so, I'd present it to your DS as what you think is best, then ask him if he has any objections, or would rather do it differently.