No need to tell him that as long as he was truely poor or needy.

I gave zakaat money to my uncle because he was collecting money in order to pay for his surgery. I gave him money but did not tell him that it was zakaat money. He already used the money to pay for his surgery. Was it compulsory to tell him that it was zakaat money before I gave him the zakaat money? Should I give zakaat again and ignore the money I gave