Something changed to Chess rating since COVID?

So my son stopped playing around beginning of April.

At that time his rating fluctuated from 1400 to 1450. He never went over 1450.

Then he stopped playing because he wanted to do some music, this was beginning of April.

He started to play again like 2wks ago and he never has never ever played/read/nor watched any chess related stuff
between April to November.

Now his rating is just little over 1600 and fluctuates from 1600~1640.

I'm just confused.

no he doesn't use engine...*sigh...(I'm pretty sure there's people who would love to comment "it's because he cheats"....)

rating are relative and pool tends to drift to towards 1500 as long as there is good influx of people. and Covid-19 definatelly caused influx of new players and apparently weaker than exising pool.

Second obvious thing is that your son in now stronger player. It does happen that people get stronger