I think the answer is punishment and more time with your 9 year old. That’s unacceptable behavior. But yes, he probably wants more time with you.

What’s his blood type

I agree with 15:36.

I would. I like the idea of writing a letter with things he likes about his younger sibling.

I would also give the older one more time and responsibility. They crave recognition of their abilities at that age and will do underhanded things to get it if needed.

15:36 again. Also if you're coming here calling him a jerk, then he knows you feel that way, so I bet you have also been making things worse. I'm saying all of this with kindness.

15:36 here and also ask him why he did it. Maybe ask if it's hard being the older sibling. I don't think punishment as such is the right move here. You can require kindness, but there's a reason he did this. You don't want to make your presence and love more conditional in the meantime.

What else has he done and what have been the consequences?

It's difficult to answer without knowing this.

For now, you can say, "In this family, we treat each other with kindness. This was not kind. It was mean and hurtful. I'm upset by your behavior and need to think through calmly about what your consequence will be."

This is a cry for your attention. He may think the younger gets all your attention and it may be somewhat true if the 6 year old gets more help even if the 9 year old doesn't need the help like with coats or pouring drinks or whatever.

I would set aside some reliable time to do something specific with the 9 year old weekly or whatever works. Like maybe one older kid board game.

How is this even a question?

This is some next level stuff from your 9yo.

Seriously apologies are in order, but also, that feels like sociopath behavior from your kid

Yes. Mean and manipulative.

Yes 10 strikes with a wet noodle

I'd make him assure the younger kid he wrote it and was wrong, then write an essay about why impersonating is wrong and how what he did was hurtful and what he will do in the future. He can not do anything until that is done.

I'd just make DS9 write another letter with (legitimate) things he likes about his brother. When my DD7 was saying unkind things to her sisters she had to write a paragraph to them with kind things. Haven't had the same particular issue since plus it helped her sisters feel better.

DS (9) just wrote a note to his younger brother saying: "Dear Larlo, I hate you. Love Mama" and my 6 year old came to me looking sad asking me if I wrote it.

I'm furious.

He's been acting like a jerk lately, and I've been giving consequences, but it's not making him behave any better.

WWYD in this case?