I know it's probably been said in this thread before, having 19 pages and all, but banning guns isn't the solution. Teaching safe gun handling is.
And the gun should never have been left where he could get to it, whether loaded (as it was) or unloaded. Guns are not toys. My father taught me to handle guns when I was a teen after he retired from the AF and I'm now 53 with four grandchildren. For a five year old, the gun should have been put up until a responsible adult could supervise and teach him gun safety.
And water guns just teach a kid that they can point a gun at others and 'shoot' them, it doesn't teach them that bullets will kill if shot into a living thing.