Sounds like my 5 year old boy. He usually does it when he's especially excited about something such as this weekend he wanted to spend his birthday money and kept asking.
Then just this morning, I told him an old friend of mine was coming over and bringing her 4 year old. My son has seen him once when he was a year old. He said to me. That boy is going to say "oh hey I remember you!" And I told him no, that he was only a week old so he wouldn't remember.
Then he said the same thing in a span of an hour 4 times! I finally told him that he wasn't listening to me and if he asked again he'd have to go to his room. (because it was driving me nuts)
When my husband got home 20 minutes later I told him about it. Then my son said. . . Mom, can I just say it one more time? And we both at the same time said NO! LOL
I can be a bit OCD in my thinking so I know it's hard sometimes when you have something stuck in your head. Overall it doesn't really effect my life, except if I'm getting to worrying about something. I went to a counselor and described all my symptoms and he was not willing to label me as OCD but said that we all have those tendencies at times.
I would think at 10 you could explain to him that you've heard enough about it for now and that if he asks again he won't be able to do it. But let him do it as soon as the weather permits. How wonderful he wants to help out!!
My 5 year old just helped me mop my kitchen and had a ball. :)