If he wanted to do that he’d have pushed his AG to deschedule it entirely
The attorney general doesn’t set the drug classification level, that’s the DEA. This is power was established by a law passed by congress, so if Biden had his AG try to overrule that some states would sue and the Supreme Court would clearly side with them. He could have his AG refuse to prosecute any cannabis crimes, but that would immediately be undone the next time a conservative AG takes over and prosecutes everyone who got a pass now. Also, they’ve already been ignoring any cannabis prosecutions where it’s someone following state law.
Apart from congress passing a law legalizing cannabis (which is not possible with the republican house, not to mention most of the senate) having the DEA reschedule it is the only “permanent” path forward. And it has to be done to the letter of the law because you know some republican states will challenge this move.