I agree with the prior posts, the red flags are going up all around you:
1. Does not socialize
2. Does not like school and is failing
3. Escapes into videogameland
4. Has already run away once
Seek professional help immediately. Hopefully it is something simple having to do with those dreadful teenager years, but what if it's not? You don't want to wonder "what if" later on. He is calling out for help in the only ways he knows how. SOMETHING is scaring, bothering him. You need a major intervention now.
You are a good parent. You noticed the warning signs. Don't sit by hoping this will go away or resolve itself. Fight for your child now. Fight for his future.
Don't blame him. Let him know you are scared and worried about him. Try to get a verbal agreement from him that the next time he is stressed out he will come talk to you. If needbe, have him sign a "contract" that says he will not run away or do anything drastic without coming to you first. (I have heard this is very effective with other addicts/depressed people. Once they sign, they have made a committment to someone else.) You don't have to be a "teenager" expert, just a concerned mom that wants some piece of mind that he will come to you. You have to remain open minded. You have to let him know he can say anything to you without being judgemental or punishing him. He needs open lines of communication right now.
If need be, call your insurance company and get a referral today. Or call a depression hotline to get referred to someone. Don't wait - please.
HUGS and good vibes sent your way. Let us know what happens.