Definitely make sure that you talk to the pediatrician about it, as it may be more complicated than a desire to walk on his toes. You want to make sure you get any potential problems diagnosed early so that treatment is less complicated. My cousin went through the same thing. She was always a toe walker, and it turned out that her muscles had developed in a way that she could only walk on her toes due to the way that my aunt and uncle had positioned her walker (I figured out that my aunt and uncle had set her walker too high for her, where she had to use her toes instead of the flats of her feet, when they put my son in the walker and raised it to toe level one time). In fact, the muscles on the front of your legs that help your foot flex as you walk weren't even developed at all. They had two younger children who both started walking on their toes, as well, but they were able to break them of the toe walking by about 16 months of age. The oldest was about five before she finally had the problem diagnosed and corrected, which required surgery, casts on both legs, and physical therapy. If they'd started looking for a reason, rather than figuring it was a stage, much earlier then the treatment wouldn't have been anywhere near as invasive.
I hope that didn't scare you. It was meant more to show you that you really need to make sure the doctor checks it out. Don't let the doctor insist it is a phase, but instead check the necessary bases just in case. It's so much better to be safe than to be sorry.
Good luck!