Did you do his side quest? If so, did you give him to the brotherhood?
It is possible his AI is in a glitched state and if you gave him to the brotherhood it is possible his weapon is missing.
The first thing to try is talking to him and firing him and then rehiring him.
MAKE SURE TO TAKE ANYTHING IN HIS INVENTORY BEFORE YOU DO THIS OR YOU WILL PROBABLY LOSE IT Also, create a separate save file before attempting any of the following.
To fix the first problem open the console (press "~") while he's visible and click on him.
Type "disable" and hit enter and the "enable" (make sure you do not close the console between those steps or you will have made this a lot harder on yourself.)
If that doesn't work (ie, if he's not now following you) try repeating it with a third command in between disable and enable-
disable
resurrect 1
enable
If that doesn't work try doing it without the "1" after resurrect. This is the least preferrable method since it might also cause him to lose any upgrades you've given him.
If he still isn't attacking anything OR if you did chose the brotherhood path for his sidequest you will then need to use this command while having him selected in the console:
additem 1694df 1
That gives him his upgraded blaster.