You could try the above, and even though I haven't tested this, it would most likely be the option you are looking for.
try this:
dude selectWeapon "LAWLauncher"
or
dude selectWeapon "RPGLauncher"
With this in the init.sqs or activation field, the unit should switch weapons, (but you might need to give him a little time to do so if you do not want the user to see him switching weapons).