When did this hamster morph into a gerbil?
Increased drinking can be a sign for a lot of things (thankfully it's a he not a she as that's one less serious problem) and definitely take him to the vet if you are seeing other symptoms alongside this. It costs about £20 for me to take my hamster to the vet but you can always ring in and ask before making an appointment.
Make sure you observe him quite a lot before you take him, so you can give the vet as much help as possible, that way you are less likely to waste money in the long run trying to work out what is wrong / if there is something wrong.
My hamster doesn't use her wheel as much anymore - she's 4 and a half months old. So it may be another symptom or a coincidence, as from my experience younger hamsters tend to use their wheel a lot more to start with.
As for diabetes, have you noticed any weight loss as that can be another symptom? Also have you moved the cage recently or anything that may have caused stress? The test for diabetes is really simple.