Well- I haven't had to deal with exact situation with my kids, however I know I always feel better when I plan for situations ahead of time. Maybe setting up a fire safety plan with your boys "just in case" will put your 5 yr old at ease. Make posters, do fire drills, practice till you have these things down as a family. This way, your boys can feel more in control if something did happen. Maybe a trip to the fire station so he can learn how fast the fire fighters respond, and learn some more safety tips (plus boys love playing on the engines)
My 8 yr old seems to get fearful easily, and when we plan things or if I show him myself he seems to lighten up. Hope things go well! Good luck!