D.M.
J.,
Okay, so here's my thoughts...what time does he take his nap? If it's late in the day, and thus why you are putting him to bed later he may not be able to get quality sleep because he's just too tired when he does fall asleep. Like Susan wrote, overtired kids may fall asleep but will have trouble maintaining good sleep at night.
I had to adjust my son's nap to suit his sleep needs...so, he naps at 12:30pm after lunch and I don't let him nap longer than 3pm then, bedtime routine starts at 7pm and we're in bed by 8pm and he's usually able to get himself to sleep.
Here's the thing though, if he's been sick...ALL BETS ARE OFF. My son is sick right now (Croup...yuck) and we stick to the same routine, and don't falter. Even if he doesn't fall asleep until later we maintain the staying in bed and relaxing until he does doze off...same goes with the nap. Even if he doesn't sleep it's important that he rests.
It will take time for him to regain his routine, and that's okay. Just be patient and understanding, and guide him back to the routine slowly and with consistency. It takes the body almost two weeks after being sick to completely recover from the effects of the cold/virus.
Good Luck!