"I think he's just really enjoying it," Bowman said. "He enjoys the training and being physically fit. He just kind of wants to see where he's at. It's more really for fun. It's been nice for me to see him swim just for the joy of it really."
Phelps has already entered the remaining Grand Prix meets in Charlotte, N.C., in May and Santa Clara, Calif., in June, although Bowman said no decision has been made on whether he will compete.
Depending on his early results, Phelps could compete in the U.S. National Championships in August in Irvine, Calif., where the team for the 2015 world championships will be selected.
"I wouldn't say it's 100 percent on the radar," Bowman said. "After Mesa, we're going to sit down and talk about it."