if the fence is yours. He cannot pant it or attach anything to it without your permission. He does not have any rights or obligations regarding the upkeep of the fence.
However, if he wants to paint it - great - it's not like you have to look at his side, and painting it will probably make it last longer, If you want to paint his side you will have to ask him to go on his land.
Re plnts tied to it - he can't really, but do you care that much?