He has practiced with a metronome for years and he's gotten very good at keeping in time with a click; this is a problem that he has in playing freestyle without assistance of that sort, so I don't think using a metronome or click at this stage is going to fix this.
This is the root of the infamous Jeff Berlin controversy: playing along with a constant reference (metronome, click, recordings) doesn't mean you have good time, you are just used to playing to a reference.
I'm sure many drum instructors have their own exercises, but one I was taught by Ed Friedland (it's now in one of his books) is to set up a metronome or other click reference to play at 1/4 the actual tempo. So there are four variations CLICK-2-3-4, then 1-CLICK-3-4, then 1-2-CLICK-4, then 1-2-3-CLICK. The player should play something simple enough that they do not need to think about it (a boom-bap 4/4 rock beat is fine) and concentrate on landing dead on the click every time for 5 minutes in each of the 4 cases. Moving the click off the 1 is where things really get interesting. This will at least demonstrate to the player whether or not they can hold steady time. Once the player is solid with playing a simple pattern at a slow tempo (say 60 bpm), the tempo can be increased if necessary. It's an exercise that can be revisited over the years just as a check-up.