I can understand your concern. I was an advanced student and I have two children who are honors students, my 17 yr old, who is a junior, just qualified for National Merit. She also had some trouble in her freshman year and went from a 4.0 to a 3.5. In her sophomore year she was in an honors calc class that was too high a level and actually got a D - her first ever. There are a few things you have to keep in mind: your job is to raise and educate a confident fully funtional adult, high school is a big change and many kids need time to really adjust, communication is the key and bright kids think differently.
You want what is best for your child based on your understanding of that, make sure the two of you are in alignment on what he wants and what his interests are. At this age those are likely to change many times, allow him more responsiblity each year, where appropriate, discuss options and possible outcomes and let him make choices without judgement. I've found that driving is a good time to chat, no one can walk away, you are focused on driving and so you can't get too reactionary and kids seeem to relax in the car. Share some of your experiences - how did you really feel starting HS? Did you have some teachers you didn't like, some you loved who made the subject come alive? Keep it positive but realistic, he's smart enough to know when you are BSing. What it really comes down to is either you've done a good job (and it sounds like you have), or not. Increasingly, it's up to him. He needs you more than ever now but in a different way. Talk,talk,talk -communication is the key. Tell him, not that you expect straight A's, but that you expect him to do his personal best. You set the standard,take responsiblity and show him how an adult functions- no excuses - explainations, what can I do differently are ok. What happened, who is to blame, is only useful if it allows you to figure out how to solve the problem.
If he is as smart as it sounds, he will turn it around but it won't be because of nagging, punishments, or desperation on your part. He wants to please you but he is also growing up and needs some distance and the opportunity to discover who he is going to be - he will do some exploring, help with avenues to keep it positive. Stay positive, state clear expectations for personal best and them you live the example.
My daugher, who got the D in honors calc, recently got a letter from Yale saying that based on her test scores she should qualify for addmission and based on income would get a full ride. If I had freaked out and dumped on her, she might have shut down and quit. Instead we got her into another level of honors calc and I built her up - "I had a tough time with calc at first too, had to take the class twice. You can do it if you believe you can, you can handle this", but she was the one who did it! Hang in there and keep it positive, this is just a bump in the road.