If he seriously intends to do this he should have mobilized them already. There will be more than 100,000 of them in that crowd by the time it reaches the border.
Stationing soldiers on the border doesn't mean anything. They can either pose there with guns or shoot, and they're not going to shoot. The crowd will rush the border. Even with the military, we don't have the manpower to chase after all of these parasites. Once they get in POTUS will be powerless to send them back. They will have to be processed through the immigration courts—which are already overwhelmed. Cases taking years to get heard Sessions' effort to speed things up have been extremely feeble. If the Caravan gets in they will never leave. And POTUS doesn't have the votes in Congress to change the immigration laws.
I don't see him having any other option than to completely close the border. Let them pile up on the Mexican side like zombies.