Poor little guy!
I would schedule a tour of the firehouse for him. Firemen are awesome!
Yes, the sirens are loud and scary, but they are used so that people know to get out of the way in case of emergency....they aren't bad guys going after anyone.
My son was fascinated by firetrucks. He still is. Luckily, a guy I went to high school with was a captain and he let me take my little boy for a tour. They showed him the pole and the gear and and how the trucks work. They gave him little books about firemen.
My husband and I were separated when my husband took our son to visit Grandpa one weekend. Grandpa wasn't hungry and didn't want his lunch. He said he wanted to lay down. He was in his 80's so they took Grandpa in and tucked him in for a nap. Grandpa didn't wake up. He passed away.
My husband called 911 and the first responders were the firemen EMT's. My husband tried to shield my son from Grandpa being taken out of the house, but let me tell you......for years after that, my son was convinced Grandpa was riding around in the back of a firetruck. He wasn't quite 3 yet. So he waved at every fire truck thinking Grandpa was riding around with him. It was sweet, really. In my little town, every Christmas Eve, an old fire truck drives through the streets with Santa and they make sure every kid gets a candy cane and gets to see Santa. It's done by the volunteer fire department. My son is 14 now so he knows Grandpa isn't on that fire truck. They've known my son since he was a baby and we get Santa curb side firetruck service every year.
Like I said, I would contact the fire department that you live close to and tell them your son is afraid and ask to arrange a visit for him. He will find out very quickly that firemen are our friends and they care about us and keep us safe. They don't "come after people", they save people. You might want to even explain to the firemen to give the same talk about police officers....sirens don't mean they are going "after" anyone. The sirens mean they have to hurry to help someone who is sick or maybe had an accident.
Fireman and Police officers don't go after little boys. They protect children.
Ambulances have sirens because they are in a hurry and it lets other cars know to pull over and get out of their way because someone might be sick and need somone in a hurry.
Sirens are loud and they can be annoying, but they are for a reason that has nothing to do with coming after a little boy.
I strongly suggest you let him meet some of these people so he can see them as helpers and heroes as opposed to fearing them.
I think it will help a lot.
Best wishes to you!