Clear urine just means that it's mostly water, rather than the urea that makes it yellow. Diet soda is mostly water. If he drank a lot of fluid today, then his urine would probably be more clear than usual.

Urinating 7 times in an hour and a half is a lot. If that continues for several days, call his pediatrician and see if you should come in. Does he have any other symptoms? Like increased thirst?