First off: It's great for you (both parents and son) that you got a diagnostic and started medication. Medication is a tool that's there to help you with something your body can't handle, you shouldn't feel any more ashamed of it than you would feel of glasses, contact lenses or hearing aids. Starting meds is a step in the right direction, seriously, and taking them will help him with developing proper habits, discipline and methods to work around the problems he will face because of ADHD. If he starts with that now, he will grow up to be a perfectly functional adult. Or at least as long as you don't expect him to not butt into conversations on demand

Even if it's nothing to be ashamed of, it's normal that you're afraid regarding meds since you're dealing with something new that represents a big change on someone important to you. Don't panic. Trust proffessionals, seek as much help (be it psychiatric, psychologic, specialized parental counseling, anything) as you need and can afford without any shame and be comprehensive with your son. Don't lose hope if meds don't work or have adverse side effects at first, and keep trying. ADHD is more of a collection of symptoms that cause a disorder and less of a monolithic "dysfunction" whose workings you can trace perfectly, it works on different axes and it affects everyone in different ways, so it has different ways to be tackled. The only medication that actually clicked with my brain was the fourth one I tried, and that was only after increasing the initial dosage. It may be a slow process for your son too, but you'll eventually find something that works.

I'm not going to say that it's going to be easy because adolescence never is (even for neurotypical people), so as they say: hope for the best but prepare for the worst. You will probably argue a lot. He will probably reject most of what you tell him and do or believe the oppossite out of spite. You will probably have a very hard time making him obey you. That's just what most teenagers are like (and pretty much what they should be like according to biology, to be fair), you will have to bear with it and there's no cure for being a teenager. Him having ADHD might make things a little harder on both of you, but it's going to be mostly the smaller things; the bigger things are just part of growing up.

I say that because the worst part of ADHD (for the person suffering it) is usually the baggage that comes with it: feeling different to your peers, feeling like you're not doing enough, feeling like you're a failure because of the expectations of others (usually your family), believing that the disorder is the source of any and all of your problems... all of that might develop into big issues down the line, so it's important that you as a parent know the "do"s and "don't"s in order to burden your son as little as possible.

If you were already doing and or knew already any or all of that, then that's great! I'm sorry for assuming you didn't, it's just something I just can't stress enough.