Sorry for the long post, this is a bit of a passion of mine, coming from a more general music ed background of teaching basic music skills to kids. Feel free to ignore, I know there are loads of drum teachers who disagree with me, but I'm happy to die on this hill.
If he can't hear it at the drum set, he won't learn it at the drum set. Stand him up and have him clap to songs, stomp his feet, count out loud. Hearing time is a fundamental musical intuition and it's very physical. It comes from active listening and participation in childhood, so not everyone is going to have it if they weren't singing and dancing a lot as kids. And if they don't have it, demonstrably the quickest way to get it is to fast track them through what they should've been doing as a child. Just basic old reliable Day-1 Kodaly & Orff stuff.
I do body games a lot with really young kids and hopeless teens/adults. March in time to the beat, right foot leading. Clap with every step, now clap with just the left step. Split the body and slap each thigh as you step. Do the same but slap the right leg on every step. We do our step first the we add saying "right left" and/or counting to four.
By the time they're marching in time, slapping the right leg every beat, left leg on 2 & 4, and counting, it's resolved forever, and if often doesn't take more than one lesson for adults, usually half a lesson and a reminder to do that whenever they listen to music or walk down the street bobbing their heads.
It feels weird not teaching drums at the drums, but as a method it works. I've gotten dozens and dozens of 4-6 year olds independently playing a money beat within 1-3 lessons this way (edit: eh, some take 2-6, but we intersperse it with other none coordination stuff). This last year I picked 3 teen students who had each been taking drum lessons for 1-2 years and still couldn't hear a pulse let alone a backbeat, couldn't count, identify a downbeat, nothing. All 3 got it fixed after just 1-2 lessons of doing stupid goofy marching games together to fun music.