hypnosis works in SOME ways, and other ways such as mind confusion. But making something specific such as making him think he's a 3YO girl is troublesome.
You have to consider things that add counter-realism to the spell.
Example with a RL situation: someone doses me on LSD, instantly I start seeing rainbows and unicorns. I am naturally miserable, like Kristen Stewart. LSD tricks the mind into thinking these things are real. The only counter I would be able to think of is to either A, kill everything in site. Or B, sit still and think of a traumatic situation. Well... killing the things visible may in turn cause a crime. If I sit still, nothing is bothered and the traumatic memory lowers my dopamine making me less liable to see the delusions.
As for the mind trick,
1) he's a male, thinking he's a female
2) age difference.
I'd say that if this was successful, it would only work for 3-10 minutes before he realizes something.