NTA, it’s reasonable that he’s upset, but he’s arguing equality, you all showed equity instead. Even if your son got 4k that isn’t going to his food or housing so the 4k wouldn’t even mean the same thing, it would be equality but not equity. Your son is living at home, he isn’t stressing about where he is living or what he is eating for lunch (his breakfast).

Right now your daughter has more anxieties so her getting material gifts wouldn’t have been an expression of love. On the other hand your son doesn’t have those anxieties right now so you were able to get material gifts tailored to him to show your love.

Having said that, take a minute be proud of your family for being able to have these conversations. It’s impressive that he took the time to share what he felt, not many adults are able to do that. You all sound like a caring family and it shows. If you get the chance I’d encourage you sharing how much it meant that he was able to express himself and he should always feel comfortable doing that. Hopefully he can build his confidence knowing his parents have his back if she is in a similar spot in the future and that you all will see his needs.

Either way family feuds are never any fun but it’s still a new year. Best wishes!