Many phones have this option. Save his number as a contact and add it to a block group. The specifics depend on the actual phone, of course.

Another idea if this is important to you would be switching to Sprint. They are now embracing Google Voice network-wide. Google Voice gives you much better controls of when, where, and who you receive calls from. http://reviews.cnet.com/8301-12261_7-20046221-10356022.html

Use Google Voice. Then, it's as easy as blocking email spam.