He trusts you enough to do this...however, fish have a protective "slime coat" which helps protect against infections. It's best not to.
But, that does not mean you cannot interact! If you wanted, teach him some tricks.
Also, if you have your hands in the water wash your hands (usually no soap to avoid soap traces getting in!) before, and after
that way no bacteria you bring with your hands (school, work, friends, other animals, etc). Just as a friendly tip!