S., you're getting some posts saying basically that your son is playing you by claiming to be depressed and that the "suicidal" or depressive talk is just a cover-up for normal teen hormones.
Wow. What a huge assumption for them to make.
Get him evaluated pronto (I would not wait). You have NO way to know if he is just making up the depression to get off the hook as some here indicate, or if he really is depressed. You have to assume for now that he is, and you need to get him immediate help (as well as immediate discipline that includes doing whatever you must to ensure he cannot sneak out again). If you assume as some posters here do that he's just playing you, and it turns out that he was indeed depressed or even suicidal -- could you live with that?
I would also check all your son's online activity and texts if you don't already. I would meet with the girl's mother immediately and tell her that you need to know exactly how she "knows" your son is suicidal. I agree with the person who said she could punch this mother for not calling you the instant she heard your son was supposedly suicidal. (I also wonder why she did not answer urgent knocks at the door in the night? Wouldn't anyone, wondering if it was the cops with bad news--? If she was not in the house, that's your huge red flag that this girlfriend is unsupervised and this house is not a place your son should ever be.)
Your son might need immediate therapy and a real diagnosis -- you don't say he actually has ever seen a doctor about depression (and surely the meds are a factor?). You and your husband might want to talk to someone about how to communicate with him better about his depression and about the girlfriend.
If it turns out he admits that yes, all the depression/suicide stuff is just him seeking attention from the girlfriend or from you -- then you and he still have a lot of work to do, probably with a counselor.