hi mate, i´m gonna pm you once i´ve written this, but just to say whatever you do dont allow this dog to be put down, and certainly don´t let this sick f*cker near it with a gun, its him who has some problems not the dog.
this is almost certainly a case of the dog protecting the child, my 8yrs old boxer is the same with my girl and the breed is well known for being this way.
its all to often the case that when something does not go "the owners way" they decide to blame the dog and put it down (not saying thats the case with you) owners need to look at themselves a little more first.

if the collie is agrevating the boxer then walking alone could be a solution but does not need to be a perminant thing, just to "retrain" the boxer or actually maybe its the collie who needs some attention too.
as said before its important not to islolate the boxer, its will only make the situation worse.
anyway, i will pm now and take about what i´m prepared to do, not going to see an inocent dog killed for protecting a child.