Sorry, i forgot to mention something.

Someone his age will be the best role model. Do you know any male of similar age or a little older who has had a mental illness? Perhaps you could ask him to get him to get in contact with your son, and they could go out bowling or to maccas or something to get to know each other a little. If he finds out that this person has had a mental illness they might form a friendship and he might open up to him.

Some organisations actually have a buddy system, where people who have been through mental illness and recovered, become mentors for those who are currently sick. (Isn't that beautiful?) Investigate this. You will have to be careful that he gets involved at his own will though, otherwise it will look like he is doing 'just another thing his parents want him to do'. To avoid this, let this person contact him, perhaps on a mobile, and make it look like a predominantly social thing, external to the family and you as parents.