While at the fountain, Eric thinks over what would happen. He thinks about if he would be 15 again, or would get even younger. Eric then decides to remain as a ten-year-old so he does not have to face the potential of growing even smaller. It is also because he can be 15 again in a few more years, which would take less time than it would if he were younger than 10.
This turned out to be a good decision as, the next day, his grounding is finally lifted. The first thing he does is invite a friend over, only he forgot that all of his friends are 15 and he's only 10.
It was also the last time Eric approaches the fountain, so he approves with his own critical decision to stay in his younger body. The local town identification agency has been notified by Eric's parents about what happened at the fountain, as well as his decision. His last approach at the fountain was also the day Eric is legally ten years old again.