Another 'no gun' household here too. I make a point of veto'ing any tv shows, movies or books that have them in it and our friends and families know that any toy guns that are given as gifts won't be accepted. I tend to think childhood is so precious that the longer they can go without being exposed to the notion of killing/harming people the better.

If it was up to me I think I would also try for 'no weapons', but that is much harder to avoid. DH recently bought DS a Captain Feathersword hat and feathersword which I wasn't sure about (but hasn't been an issue - it's for tickling not hurting people). And we have some fireman duplo that came with little axes.

But that doesn't help you does it. I think the suggestion of buying a water pistol and teaching him to aim/knockover objects is a great idea. If it was me in your shoes I would be trying to change the association of guns being for killing/harming people, to them being for aiming at objects.