How can I give effective support to a stuttering partner?

How can I give effective support to a stuttering partner? I don't stutter, but my boyfriend does. He gets really upset and frustrated with himself and quite often will just apologise and give up trying to say something. Can anyone offer advice on how to best comfort him when this happens? I'll say it's okay, or squeeze his hand or something if I'm holding it, but I really wish I knew what to say that would help alleviate his self-consciousness somewhat.