Can I transfer airmiles to my sons skysurfers account?
I cannot upgrade us before our flight as we have tickets that are only valid for upgrade at check in, not before.
Will he be able to upgrade at check in (I will be there)?

Assuming you are travelling with him on same flight you can upgrade him with your miles at check in but you will not be able to use a combination of his miles and your miles - only yours. Alternatively (assuming you are happy to use your miles alone ) whether you are travelling with him or not you can log in to your account within 24 hours of departure (the closest to 24 hours the better) and then access his booking through Manage My Booking after you have logged in and try to upgrade his/your booking with miles before you actually check in on line or at the airport. If you leave it until you get to the airport there are indications that they will not let you use your miles to upgrade him unless you are on same flight. You can transfer miles to his account I believe at a cost of 2.5 US cents per mile- these will be your "oldest" miles for expiration purposes and they reset to minimum 3 years life at the time of transfer. Hope this helps and does not serve to confuse.

What's the chances of upgrade at check-in? As in, if there is at least 1 spare seat, do they upgrade as of right with your miles

At check-in, seat assignments become entirely at the controller's discretion - in my experience if you get there up until about 90 minutes before hand, they would love to relieve you of your hard earned cash and miles!