OP, if your boyfriend ever goes get into Pokémon again, suggest he keep rare Pokémon like shinies in Pokémon Home when possible. Home is cloud storage so even if you delete his profile the Pokémon will still be there. I have collectively spent over a thousand hours in Pokémon games and keep my living dex, shinies, event Pokémon, etc. all in Home to prevent this exact situation from happening. Unfortunately, Pokémon Home isn't available for Pokémon Scarlet yet, so there was nothing that could be done in this situation. Home connection is coming this Spring, just in time for him to start the game again if he chooses.

The good side of that is that he didn't trade his beloved or now impossible to obtain Pokémon from past games to Scarlet because it's not possible to do that yet. There's really nothing you deleted from his game that cannot be obtained again, though if he had a lot of shinies and had a complete dex it'll probably take another hundred hours to get back to that point. I can't blame him for not wanting to dedicate that much time to redoing what he's already done.

Pokémon games are some of the few games that do not allow cloud saves on Switch because they don't want people to be able to clone Pokémon/get multiples of rare Pokémon. So if the profile gets deleted, the save file is gone. There's no fix for this unfortunately.

As someone else who is autistic, I'd recommend you step back and take a deep breath next time a change is bothering you. It can be tough to get over the impulsive response to reject change but putting up with things in your life changing is part of being in a relationship. Even if he did put a profile on your Switch, it's really not the end of the world, and not something that needed to be deleted without talking to him first.