This morning, when Jet and I got into the van, Jet climbed into his seat himself, grabbed the strap and said to me, "Mom. Do. NOT. Help. Me." between gritted teeth.
So I didn't help him. I just let him do it and applauded when he did. He happily tightened the belt across his chest and said, "It's harder when I have my big coat on." He was so proud it was cool to see.