So keeping him in Sweden requires an individual exception.

I won't address the value problem of "then why not give that exception." Instead I'll say: No it doesn't; the Brexit withdrawal agreement has provisions specifically for British people living in the EU at the time of implementation. This is factually wrong, as seen by the behavior of literally every other EU member state.

There’s no reason to grant someone who in 20 years never applied for Swedish citizenship an exception.

Setting aside the value problem of "the right to live in one's home isn't something to be granted," see above. Again this isn't an exception; it's the rule as set in the withdrawal agreement. Also this is your third time moving the goalposts and I'd really rather you commit to one line of argument or admit that you're defending the indefensible.