I just Googled it and briefly read that it seems to often be associated with having a more anxious type personality. I have a very shy five year old...new situations are hard for her and she is very clingy...I think some children just tend to have a harder time easing into new situations. I wouldn't worry too much or presume that your son actually has selective mutism (that may be a little premature for your doctor to have said), but it's great that you are checking it out.
One place you may check into is the Center for Social Success - go to www.dristre.com. Click on the first page to enter the site. Dr. Istre is a nurse who spoke at a MOPS meeting I attended several years ago. She is very good and has a staff of therapists with varied backgrounds who work with all kinds of issues. I have considered taking my child there. I have an acquaintance whose daughter has attended group therapy and has really benefitted.
Good luck and God bless!