I say don't. The K-12 program is really strict now and they require kids to be at the right age for each level at the start of the school year. You would not want your child to repeat prep before he can get to Grade school. You can start teaching him at home though with the basics to prepare him i.e. alphabet, numbers, colors, shapes, pattern writing, simple reading.
There are some preschools that accept applicants as young as two years old. If you want to get your kid socialized and ready for prep or kindergarten, play schools are a good option. You might find this article helpful: https://ph.theasianparent.com/metro-manila-preschool-guide-sy-2015-2016/