This is a tough one, because you can't honestly tell him that there is nothing to be afraid of; a real fire can cause death and he is old enough to know it. Kelby's right, he sees something that is out of his control, and he knows it's out of your control too. Especially since he remembers the fire from last year. I think that the best thing to do is to show him that you take his fears seriously.
It does not shock me that he wants to sleep with you and has peed his pants. It's going to be annoying for a while, but let him sleep in your room until he feels safe in his room again. If you don't want him in your bed, then put his mattress on the floor in your room.
Don't chastise him about it, and don't tell him "this is only temporary, you need to be in your own room again soon" because he already knows that it's not a forever thing. Let your son hold your hand until he is ready to let go, don't do the letting go first!
And about the peeing, I don't think that you should put him back into diapers, that will send him the message that you think he is a failure. He'll find his way back to the toilet. Probably just reminding him every hour or so to use the toilet will suffice.
Help him to face his fears. Try taking him to the fire station for a tour. Tell the fire fighters in advance (on the sly) about your son's situation. They will take that in consideration and explain to your son how they keep people safe from fires. Before you leave the house, ask him if he wants to go. If he says no, then gently say "maybe next week, if you change your mind, we can go".
And finally, post a note near the fire alarms in your building and the one next door, for people to stop pulling false alarms, for the sake of your little boy! That has got to be driving you crazy!
Hope my advice helps.