J.R.
I agree with Deb K. You should talk to the daycare and find out about the nap environment. Do they at least keep the environment quiet and dimmed? What about their nap routine? You want to keep your routine at home the same as at daycare. Perhaps your prenap routine is different at home than at daycare, which is making it hard for him to relax at school now that he's older.
Also, some kids need more winding down time than others, especially in such a stimulating environment as daycare. I know the older my son got, he more winding down time he needed, BEFORE the nap routine started. Perhaps before they begin nap, they can have him draw?
You do need to think about whether he really needs two naps. He may just need some quiet time. I know you said he naps twice on weekends, but sometimes kids do that because they are more stimulated by being at home. Think about it: all kids love to be home, and on weekends, you are home too. Very exciting. I am a sahm, and my son went to one nap a day much later than yours - about 18 mo - but he does the same thing on weekends, he takes a morning and afternoon nap. He is just so excited because Daddy's home he tires himself out. It was a rough transition, but eventually we all got used to it. To help, I made sure he got quiet time. If he wasn't going to nap, we hung out in his room, with the lights dimmed, or we colored quietly, or read books. Something quiet and calming. Most days, he took a morning nap and had quiet time in the afternoon. That's usually all he needed -a break from the stimulation, and then he would be much less grumpy, and able to continue with the day. Now he has an unreliable nap schedule, but still just one a day except weekends.
Also, some kids are afternoon rather than morning napppers. If they absolutely cannot get two naps out of him, ask if they could try keeping him awake in the morning, but in a less stimulating environment, and have him nap in the afternoon. My son is a late morning or LATE afternoon napper. He sleeps from 10-12 or from 3-5, and still goes to bed around 8pm. Whatever works for you, but you need to make sure he is getting 13-15 hours of sleep, however you can get it out of him. It is important for his mental and physical development.