I don't know if I agree that 210 at 5"9 would be his natural maintain weight unless seriously broad/muscled. I was 210 at 6"0 while doing mandatory PT and moderate lifting and still was pretty chunky.

I would definitely try IF and cut out everything to just water and basic foods to see if its something sneaking in carbs or spiking you out of ketosis. Once you are comfortable and see that you are solidly in ketosis and losing again, then you could slowly reintroduce everything and monitor it.