For the military to build the wall, that would qualify as a military construction project, and there is currently not anywhere near $5 billion appropriated for that. So, if Trump decides to use the military, he will have to pull money already set aside for another project and use it for the wall. However, to do that would require approval from Congress... So, once again, you are screwed.
This is how our government works. A President can't rob Peter to pay Paul around here without getting approval from Congress.