He may be able to instruct the ATF, etc. in what their policies should be but he cannot enact law via an EO that requires the states, cities and counties to comply should they choose not to.
He could shut down the instant check and make it a mail in check. And then cut the staff.
He could do a lot to drag this out.
He could revoke or not renew any FFL.
There is a lot he could do.