Our first advice would be to find a local trainer in your area who can work on the issue. But, there are other things you can do at home to help your canine best friend with his reactivity issues!
What’s reaaaaally important during this timeframe is to make sure that you’re teaching your dog what you want to see more of. If someone rings the doorbell and he doesn’t bark, praise and reward that calm behavior! Make sure to communicate what you want by rewarding him for it. Don’t just take it for granted.
For example, last week I saw a dog grab a treat out of his owner's hand with an intensity that was not ok! She laughed, but I knew that if this wasn’t addressed, it would become something worse. And because she was in front of me, I was able to see and fix an issue she didn’t even know she had! That’s what class (and private lessons) are for.
They’re watching their dog being all excited and think “Omgg, my dog is so happy, look at him!” but, what they don’t notice is the OTHER dog, who is TERRIFIED or UNSURE and giving very clear body language that’s basically saying “Get me out of heeeeeere, I’m dyiiiing!”
Older kids can also volunteer and walk the dogs at your local shelter. This is a great opportunity to get some hands-on experience AND give back to the community, just be sure that the organization is giving sufficient training to everyone before they start handling dogs.