It’s been tried before and he won’t win the fight. The military provides “Law of Armed Conflict” training that explains the circumstances when you can refuse to follow an “illegal order”. It’s a fairly narrow scope, such as if you’re ordered to shoot non-combatants.
There were a lot of draftees that tried to get out of Vietnam claiming that it was an “illegal war. The same for Korea. Neither one was declared by Congress. I am unaware of any of these people who succeeded.
Yes. If POTUS decides to invade Canada and annex Vancouver (just for fun), there is nothing soldiers can do, except follow orders.
Only very specific narrowly defined acts, like direct order to execute civilians can be rejected.