We have had this problem for 6 months now, in two bands as it is the same drummer. I felt it had to be mentioned a while back but the guy had just lost his father unexpectedly so it was decided to say nothing at that point.
It came to head when a couple of pros who were friends of our French guitarist both told him that it could be a great band if we changed the drummer. Our guitarist politely tried to tell him we had a problem (slowing down, sometimes before the intro had even finished) and basically he was told the HE was a guitarist, and I'M the drummer etc etc.
A common problem, and I've only ever found this with drummers, is that they think because they've owned a kit for 15 years, they're good drummers.
Anyway, I decided to go for it last week and confront the problem. (Just me and him) I did this using politcs and lies basically by telling him WE had a problem as a rhythm section and WE are slowing down. (I'm no Randy Hope Taylor and am aware of my shortcomings as a player but my timekeeping is good) Anyway, it worked. I gave him a spare metronome and he used it at rehearsal. I taped it to the hi-hat stand. The difference was amazing, not perfect as it takes getting used to using a met whilst in a band situation but we're well on the way. I have had a unit built that plugs into the headphone socket and sends a signal that lights up 4 white LEDs on a separate unit so the whole band can keep an eye on the tempo.
These problems are common and the main reason they don't get solved is the friendship and feelings issue. No-one likes to be criticised about their abilty and it's a delicate situation when a band has got together for the sole reason of having fun.
Personally, I welcome constructive crticism as I see it as part of the road to improvement.