If he was up at 6.45am, I'd probably aim for a nap about 10.15/10.30 (so about 3.5/3.75 hours A time) as he may not handle his new A time given the nights sleep he had. Easier like this:
6.45 Up
10.15/10.30 Nap
12.30-12.45 Awake
3.45/4pm Nap (for 45 mins-1 hour)
7/7.30pm Bed
Although I think you'll have to judge that first A time, if he can't make it that long, do earlier nap and still let him sleep until 12.30/12.45.
How does that sound?
Just watch him incase he's very tired because he's sick. Have you his cot elevated a bit, can really help with breathing when they are all congested. We've had weeks of restless sleep here with a very bad cough so totally know how you feel.