At least his fear of zombies is rational, kinda.
Kids can be scared of strange things. My son (5) is fine with RZSSI, but was not a fan of the ghost cow on the AMH backglass. I was able to fix that one by making it silly rather than scary for him. I told him it said "Moo boo".
A nephew was scared of the wolfman on MB when I owned it. He's like 12 now, and he still says he's glad I sold it.