Playing only long formats

Is there a flipside if one wishes to play only the long format of the game? My son (9 Yo) used to play blitz, rapid and classical until a few month back. He suddenly felt he doesn't want to play blitz anymore because he isn't able to handle the time pressure and switched to Rapid and Classical. He did a good job in both but now he is starting to have time control problem with Rapid. He wants to now restrict himself to playing only the classical. But, he plays classical very well. (lichess.org/@/pranavramesh8/)

Should he work on his time control and continue to practise shorter versions of the game ? Or does it have more to do with a person's style and he could quit playing the formats that he isn't comfortable with?

longer time formats have always been better for developing your chess skills. It helps with calculation ofcourse.
The only 'flipside' i can think of is when he tries to adopt a new opening it will take him longer to get to know all variations of his opponents replies. You could play 100 games of blitz and learn the opponents patterns quicker then in a few games of classical. That being said i still think longer time formats are more beneficial to him. I taugh some chess at a chess club and usually the kids have trouble focussing for so long on a single game, i think you should be happy with your kids choice to commitment.

Don't force ur kids development. He may end up quitting chess for good. Many lichess player quit every day because of losing patient/passions (both)

Classical time control is the real chess.
Rapid is still good as well.
Blitz and bullet are fun, but do not lead to improvement.
Focus on classical is excellent.

I think mixing it up and playing a little bit of everything is the best. The amount of matches you can play in a short period of time in blitz makes it very easy to pinpoint specific openings or moves that seem to cause you trouble, making it easy to find what areas you need study/practice in, and once you have studied it and played a few slower games with your "new approach", you can go back to blitz to try to just play the correct moves very fast, just to make sure you remember the ideas and moves so you don't have to solve the same problem every time you encounter it.

I think he should start playing real tournaments or team competitions. Ofcourse it's hard with a pandemic going on, but it will vanish eventually.
As for internet chess: I think having fun is the top prior. I don't play rated games, because i think it destroyes some of the fun aspects. I'm a terrible attacker, because i don't really know how to listen to my intuitivity, but on the internet you can do whatever you want.