I agree with Amanda - it all depends on where you are sending your son. Ask to see the centers curriculum, or a weekly activity plan - they should be doing music, art, free play, reading/language (flannelboard counts), and physical activity.
Also.. something to note... not all centers in a chain are created equal. A couple years ago, my son went to Kindercare in Woodridge, but I was not happy with them. I pulled him out and tried a couple other solutions (nanny, home daycare, babysitter). I was hesitant to put him in a Kindercare again - but we moved. He goes to Kindercare (on College St in Lisle) and the difference is like night and day. This center is very clean, the toys are all in great condition, and the staff if very friendly and helpful. He's a schoolager now and only goes before and after school, but they help with homework and practice writing letters, etc.
Even if you only send him to a daycare with a pre-k curriculum for a few hours a day, he will still benefit from it. If you can afford to leave him in longer, then maybe that could be an option. I used to work at a daycare/preschool and we had very busy days of learning everyday - some kids came 2-3x a week but stayed for the full day so they could experience everything. For that age, routine is important, so while the activity itself might be different, the type of activity should always be the same. (for axample, we had breakfast, then 15 minutes of free play, then music, then storytime, then freeplay, then art, then outside time, then lunch, then nap, then snack, then free play, then reading/language, then math (counting, etc) then outside play, then free play.... so in the course of a day, a child does many activities. If your child only goes mornings or only afternoons, he could be missing out on a lot. Every class is different and has a different routine - thats just the one we did.
Check with the classroom teacher and see what he's missing by only going the limited hours he's going. good luck!