But he's not really your partner... or a Dad... any more, if he ever has been, surely?
I'm quite baffled by your post - it's been ten years in which you've presumably carved out a completely single parent life for yourself, have friends, situations, a life that your DH has never been part of. How can you still feel that he's your husband in any meaningful way?
If your son has been grieving his lack of a Dad for so long that is tragic. I assume that's what you mean by 'not getting used to' his absence. I imagine it's very hard for him.
Your post reads more as if he's just been away for six months or something. But your son has hardly ever known different. You've lived separate lives as far away from being a family as you can imagine, for a decade.
Have you ever visited - for a decent amount of time, not just a week's 'holiday'?
I'd be astonished if your DH's life out there doesn't encompass far more than he is letting on. Suddenly when he's due to come back, whoops, he 'has to' stay longer to get a promotion?
He's not exactly pining, is he? I smell some bullshit.
I don't think he wants to come back.