Actually he can in the interests of national security.
There border is extremely porous and we know chinese and iranians have used it to illegally enter here and disappear.
He sure as sh1t can do it. Fedgov is actually tasked to gurd the borders and he directs the military. Its not a declaration of war so no approval by congress needed. Securing the borders is part of his job, and the military is responsible for keeping the country safe. We have enemies and peoples who have zero respect for our laws and borders and we need to shut this down. These people come here and infect us and rape and murder us.
Providing border security is not the same as misusing 5b in military appropriations to be built by the military. To build the wall The House must originate the appropriations. The senate must also approve.