You can still be firm, and not use smacking, but from the sound of it I don't think smacking would be terribly effective anyway. What I would do is make it very clear to him that he will start to miss out on doing things because he misbehaves too much - try not to use the word naughty if you can, but he's at the age where he should be able to understand actions and their consequences. You need to try to negotiate with him that if he does things that aren't right, like run away at the shops etc, that he wont be able to do things, like he wont be able to come with you (and for a while you might have to delay doing shopping etc when your DH is home). But the main thing is you have to be firm - when you say you are going to do xyz because he wouldn't do xyz, make sure you don't cave in because it not only sends mixed signals to him because theres no consistency, but also that if he carrys on for long enough, that you will give in. And when you are at the shops, and he would prefer to walk, get one of those harness things - either the wrist one or the backpack one so you can at least contain how far he can go. You will get some looks, but I would rather those looks than the ones you get for having a child that is running amok kwim?

When he's at home, I have found that with one of my children, ignoring her when she was carrying on was the best way to deal with it. I know some children will only act up even more to try to get your attention, but I made it very clear to her that I would only talk to her if she wasn't doing xyz and if she was playing up I would ignore her for the duration of the tanty etc. I would make myself look busy tidying up etc, just simple things, but never go and play with another child during that time kwim?

I hope that this is only a passing phase though and you come out the other side soon