Hi, P.. I understand completely. I am a special education teacher, and also the parent of an ADHD son (just turned 20...there is a light at the end of the tunnel!)
You are correct, yelling is counterproductive, as it will not change the behavior, and often reinforces it.
First, you need to find what IS reinforcing for your son.
These are some of the things you will need to use to help your son use more appropriate behaviors. (I give some examples below.)
Second, always allow time for transitions. Letting him know at 7 PM that 9 PM will be the bedtime is good, but he might need reminding at 8:30 that he has another half hour. (Eg...Jake, Remember bedtime is 9:00 tonight. If you get your PJs on and teeth brushed now, you can play video games until 9:00.) Then set a timer for 8:55 and let him know when the timer goes off, it is time to finish whatever game he is playing. No yelling needed...the timer is the judge, not you.) Then have a positive reinforcement for 9:00 bedtime...how about a "mom and son one-on-one?) This will make the 9:00 time special. Once he is in the bed, sit next to him, and you each have a chance to ask/question the other. Tell him he can ask you any question in the whole world, or he can use the time to tell you about something. (no matter how boring to you.) During this time with my sons, I learned about football statistics, robots, and had some intense political discussions. He also wanted to know my life story...Mom, did you ever....???) You may need to set the timer for 10 minutes, and if he still has more to talk about, make a note about the topic so you can continue it tommorrow night. --Be sure to write it down, so you both can remember what you were talking about. ADHD people often hate interruptions because of "losing their place." Writing it down will aleve this fear. --
This is an example of a positive way to give him attention for appropriate behavior. (He is in bed, and he is getting your positive attention...win-win.)
Since your son is old enough, contracts are another way to help HIM learn to control his behavior. Make a contract with him to make bedtime less stressful..and create it together:
Jake, let's work together to make bedtime less stressful on us both. Bedtime needs to be 9:00 on school nights, because you have to be up early for school. Does this seem fair and reasonable to you? (Yeah, mom, but I should be able to stay up later on weekends.) OK, so what would be a reasonable weekend bedtime? 10:00? So let's make a contract. If you go to bed without fuss at 9:00 all five nights, Sunday through Thursday, you can EARN the right to stay up until 10:00 on the weekend. Put this IN WRITING adn you BOTH sign the contract.
Make a schedule, (post on the fridge, for example, and then have him check off each night he goes to bed without hassle. Then he can write on the calendar on Friday and Sat: 10:00 bedtime. The caution, if he does not fulfill his contract, he does not earn the reward that weekend. This may cause temper tantrums on the weekend, but it may be necessary for him to learn that the contract is binding. Now you don't have to yell...the contract is making the decision, not you. Remind him that he can earn the reward next week.
Does he have a special ed teacher or psychologist? He/she may be willing to work with you for home behavior management strategies. A key is, since your son is older, he will need to be part of the planning, and often wants to know why. (Why is a 9:00 bedtime necessary?) He probably has ideas on what he would like to "Earn."
Caution: this is not an easy process, it will take time, but is worth the rewards. Remember that you can't control his behavior, he has to learn to control it himself. You are just helping facilitate the process.
My ADHD son is a joy now. He still is learning, but as an adult, he understands his disability and has tools to help him. (For instance, he has to write everything down so he doesn't forget, etc.)
There are agencies out there with support groups, such as LDA, that may offer you some support or suggestions. When my boys were young, I attended a support group for moms of ADD children so we could share ideas.
Good luck, and post again to let us know how things are going.