L.P.
It all depends on his temperment. Talk with your son's preschool teacher. She/He will be your best resource.
We didn't have a full day option with our first child but I know she would have excelled in it. The hardest transition for her was to go from 2 full, one half day KG to 5 full days of 1st Grade.
Full day came around when our second was ready for Kindergarten and her preschool teacher told us she was very ready to learn and would do very well in full day. She has excelled and has begun reading well at an earlier age than the first. The transition to 5 full days wasn't too difficult because summer was inbetween and she wasn't napping much anymore (her preschool time was in the afternoons).
Now, our third child is a boy, 4, and in his first year of preschool. He has one more year b/c his b-day is December, so it will be interesting to see what he will be ready for. He doesn't care much for table top activities such as coloring, gluing, etc. but will sit down and do the minimum.
Good luck!!