First, I want to say how wonderful it is that you are honoring his correct pronouns so quickly and that you are being so supportive. I'm sure he is quite relieved and happy right now that his talk with you went so well.

My son came out to me a year and a half ago just after he turned eight, so my level of support is a lot higher in many ways I'm sure. He also wanted me to tell others for him when he was not around and then report back to him who knew and whether or not they were supportive. We talked a lot about what he wanted me to say and not to say (but he is very open, so he was not very private about anything).
Personally, even if a friend of mine, whether they were 18 or 48, asked me to do this for them I would be happy to. This journey takes a lot of courage, and asking for help and support is reasonable and necessary for his wellbeing. (Anyway, one of the first things people would want to know is "What do your parents think?", so you can kill two birds with one stone!)
If someone is going to have a negative reaction, it is usually initially, and then after they have time to think about it they will often be very accepting. So you are also saving the person you are telling by giving them a chance to understand and digest the information before they see or talk to your son. If I could be that kind of barrier for any friend or family member I would gladly do it. This road has got to be scary enough, even when you have the support of the people closest to you. Unless you two go everywhere together, he's going to have to deal with plenty of questions and different types of reactions no matter how many people you talk to for him, so he will get plenty of practice.
The biggest thing that helped me was practicing saying it with confidence, and to be clear that we are fully supporting him before they have a chance to respond or start asking questions. The way you say it makes a HUGE difference in how people will react. Remember, you are not apologizing for anything. You are simply informing them of a change in your lives. You will be surprised how many people are very supportive, and others who you think will be supportive will surprisingly not be. Then there will be those who seem supportive but you will not hear from them much afterwards. Just remember that this is good attrition and it makes room for all of the new friends you are going to make through this new journey!
Please keep us updated on how you are doing!
Amy