My brother added salt to the whole soup without asking. How do I accept kitchen help without handing over the dish?

I left my brother beside the soup while I set the table. When I returned, he was putting the salt away. "It needed something," he said.

I'd already tasted it and liked it. After his addition, I found it much saltier than I wanted, but I served it because everyone was waiting. Later he told me I should be glad somebody checked the seasoning.

He often helps when I host, and I appreciate having another pair of hands. What I hadn't realised was that "could you stir this?" sounded to him like an invitation to finish it however he thought best.

Before our next dinner, how would you make that distinction? I want him to feel welcome in the kitchen, but I'd like suggestions about the whole pot to come before the ingredients go into it. He can season his own bowl as he likes.