He may not be quite ready to give up that afternoon nap. My son is 4 1/2 and while he doesn't always need to nap each day, he needs one about every other day. School can be "stressfull" and tiring. I would think that something all day would be aware of that and offer a quiet time. You might ask the school about it.
Yes, he needs to start preparing for all day once he's in 1st grade, but he'll get there and until then he may need to have that bit of rest. kids tend to burn a lot of energy at school.
Does the school offer an afternoon snack. If not they probably should because that afternoon time can be the hardest hurdle.
My brother was the same way. he could go about 2 days without a nap, but then he had to have one the next day. Watching TV doesn't really allow him to rest if he needs too. while his body is doing nothing, his mind is still going.
you could also try, making a short drive after picking him up so that he can take a brief cat nap. generally, speaking if we can get at least 20 minutes in we'll be resurged enough to make it to bedtime.