Could some cells have escaped from the primary tumor even before surgery ?
Yes. I had clear margins at surgery, received chemo after surgery and had a recurrence 3 years later - determined to be from the original tumor.
You can drive yourself crazy over all of this - try not to. Many stage 3's survive - many don't. It's all a big crap shoot. All you can do is have him follow his doctors advice, try to stay active, eat right, all that good stuff and just hope for the best.
If he had scans done in February, there's probably no reason to scan again so soon. When I finished my post surgery chemo, because I had so many lymph nodes affected (27 of 38 removed); we scanned every 3 months the first year; every 6 months the second year - and then the cancer returned. The most frequent I've seen most scanned is every 3 months - and that was to either watch for a recurrence after finishing chemo - or in my case now to monitor the chemo I'm receiving to see if it's working on the active cancer cells (if it isn't then we change chemo regimens).