Normally such a restriction is built in to the battle standard entry in the army book but in the case of the HE army book, no such rule exists.Hmm... I can't find it either, but I could swear that he can only use a hand weapon or magical hand weapon. I mean, one hand is taken up by holding the banner... but then again, the BSB can use a shield, if you look at their models they show in the army book, so maybe High Elf BSBs are special in not having equipment restrictions.