A very similar situation occured when my now 20-year-old son was your child's age.
I was so confused as to what was going on, and in the dark.
I finally found out...my son had gotten involved with drugs and alcohol at 16 and was abusing them by the time I noticed his behavior changes. He was the LAST person I would have expected that from.
Since you mentioned the grades, a change in attitude, dropping activities that once gave him joy (dance and acting), a change in a few friends, etc...I suggest the following: take your son to the doctor (you can tell him why OR you can tell him it's time for his annual checkup - that's up to you) and ask the doctor to perform a drug test (drug profile that's performed from urine collected from your son at that visit). A Monday would be a good appointment time since he may still have substances in his urine (like alcohol).
My son denied any substance use. I bought a home breathilizer...but it never showed any alcohol usage when I tested him. That just led to him screaming that I didn't trust him and he couldn't live with me if I wasn't going to trust him - which made me feel horrible...but he just kept behaving more and more like someone I didn't know and he seemed to be getting more and more out of control.
I told my son we were going for a drug test and he blew up. He raged. He pulled the trust card again. BUT I held my ground and we went. A day later I was called with the results. He had SEVERAL drugs in his body, prescription and illegal. No alcohol at that time.
I immediately entered him in an outpatient adolescent drug treatment center in town - and took him (kicking and screaming). This facility had mandatory family counseling (with the child present and without the child present), and in the counseling without the child present I learned how to spot, handle, and deal with a child who is using drugs...some things I learned I hadn't had to deal with...but as predicted, those situations surfaced later and I knew how to defuse it because I had been educated to the possibility. And we learned that these drugs have changed our child's brain...we aren't dealing with the same person...we are dealing with the drugs...we aren't talking to the same person...we are talking to the drugs...and even if the child concurs the addiction, there may not be a complete "return" of the former child because of the permanent effect of the substances on the brain.
My son straightened out and his grades improved and he made great progress...he was back on the honor roll in his senior year...paired back up with former friends...smiled and actually seemed to feel joy again.
I wish I could say he is over the hump...he's in college...but he has periods of regression - where he turns to drugs again...and those situations break my heart and my spirit - but, it's not an easy road for the family to walk. And now that he's of legal age, there are limitations to how I can "step in" when he regresses...so take positive action while you still have legal control over your son's affairs.
The gutter-talk language is a whole other matter...but do not permit him to be disrespectful...deal with that immediately and swiftly.
One action the counselors recommended was to clear their room of everything but a bed (I've heard Dr. Phil recommened this as well). No electronics. No phone. Nothing. PLUS (and I haven't heard Dr. Phil recommed this but it's effective) remove their bedroom door (put it in an attic or garage or basement, preferably where they cannot find it). Removing the door rids them of privacy - which teens cherish. They can dress and undress in a bathroom, so they don't need a door on their bedroom.
Of course, no negotiating on getting the stuff back for the first month, but later it can all be negotiated and earned back - the door being the very last thing to return. Since my child (and the teens of those at the treatment center) used drugs, earning stuff back was also conditional on urine tests. And even after they were released, parents were told to continue urine tests (and being unpredictable on WHEN the kids were going to be tested - kids clue into patterns and might not use drugs if they expect to be tested...and most kids use drugs heavily right after urine testing).
My son complained about losing stuff - but when the door disappeared he was LIVID. Then he RAGED. Then he BEGGED and cried to get the door back.
You have to stand strong. You have to be firm. It's extremely exhausting and sad and you wonder why, but there's never concrete answer to that.
As much as you want to trust that this is temporary, it probably isn't.
As much as you want this to go away on it's own, it probably won't.
As much as you hate to take action, you have to. That's why I suggested a urine test. Line it up before you speak to your child and do not let him talk you out of it. If it comes back "clean", great - you know what you're NOT dealing with...but if it doesn't, you have a starting point for corrective action.