Try going to one nap, seems about the right age for it. He may have to go down earlier for the one nap...my son started out at noon and would nap two hours. Than he moved to 1pm and naped for two hours, now he's back to noon and sleeps about two hours.
He could be having a hard time breathing, or coughing keeps him up, he jsut wants comfort and he knows if he makes a stink he'll get it. Even if he is sick, unless hes falling asleep (keep it short, 20-30 minutes), stick with one nap, consistancy is key...kids need routine and live by it. If you change his nap schedule day to day, he'll never know if he's having or two naps.
If its really bad and you know he could use a quick nap, put him in the car and go for a ride, that always worked with my son.