That would require the cooperation of Congress and the further cooperation of the judiciary at both the state and federal level. All but the most extreme of the latter believe our current libel and slander laws are the only possible versions of them that are compatible with the Constitution, and the former want to keep their jobs.
I guarantee that Congress, regardless of favor or opposition to Trump, remember how many were thrown out the door over the original Patriot Act. And that didn't even get used to suppress people!
In addition, the President cannot propose law. Only Congressprites may propose laws. While not a huge obstacle to overcome, it does ensure he cannot literally determine the contents of any such proposition.
States rights, remember? Short of a constitutional amendment anyway. That's how states have legalized marjuana despite the fact that it's federally illegal. He'd have a heck of a lot of opposition, and then there's SCOTUS, the very SCOTUS (minus Scalia) that said 'money is free speech'.
That's not how that works, federal law both adds to and supersedes state laws. The reason for legal marijuana being tolerated is basically "don't want to fuck with it". The DEA has a lot of work to do and this is usually not worth the controversy, but people can and have been arrested under the federal statute in a legalized state.
That may be one of the severe difficulties for weed soon, as Trump has made statements to the effect that he will not tolerate legalization and may send the DEA to try and suppress it. Granted, this remains far beyond the scope of operations that the DEA is physically capable of.