is this past self-harm or current, ongoing self-harm..?<br />
if it is in the past, you can wait until the relationship is established and you know him and are comfortable with him before sharing such personal, private information.. if the self-harm is still going on, i suggest putting off dating, dealing with and getting help for your own issues before you get involved in a relationship.. they say, one cannot love others until they learn to love themselves...
Well relationships require complete honesty but you can always keep things to yourself. If you feel comfortable talking with him about it then go ahead. It's nice to be able to confide in people about things you've done. When can only be up to you. Has it come up in conversation? How relevant is it?
Then I say if you aren't comfortable don't bring it up until you want to since it hasn't come up. But if it's something that is still happening I'm not so sure dating would be a good idea right now.
It really depends on how comfortable you feel with him.