Well one thing to do is ask if he wants to take him to go to his favorite resturant. See if he could take his best friend, then at the end of the day get his mom with you too, all those that is important in his life and tell him that you love him, everyone.
Seriously, take him to either your primary care Dr or a child psychiatrist. There are many reasons for depression in teens. I was depressed as a teen, and did not get the help I needed. I'm sure it affected my grades, my self esteem (because I did not socialize) and I ultimately got into drinking and smoking pot--which only made it worse. Finally when I was about 21, I started having suicidal thoughts and sought help on my own. I have a beautiful family now and those days are in the distant past. But thank God I did get the counseling I needed.