If you reply to one of his messages, it will have a unique message ID in the header, assigned by his ESP to his original message, that they will recognise and allow back through.
As to why your mail is getting junked, there could be lots of reasons at lots of points in the chain.
If your friend has been misusing the spam button on his email client to delete messages, it may have flagged all your stuff.
If his ESP is scanning incoming mail, there may be something in your mail that's ticking a box that gets them marked.
Someone else may have been misusing their spam button on your emails, and their ESP has put you on a blacklist your friend's ESP uses.
If you use a generic ESP (yahoo/hotmail/gmail/etc.) his ESP may have flagged the domain (to the right of the @ symbol), if they've had too much spam from other users.
How much your friend can do to stop it depends on where in the chain your emails are being flagged.