You can change whenever you like. In your contract it will say there is a notice period but even during that you will probably just not be able to have submissions sent out with your new agent, it won't stop you developing a relationship with him/her.
Society of Authors has lots of good advice. If he's in the AAA, you are also entitled to be given a list of who he has sent it to.
No experience Jude (I'm still looking for my 1st agent, sigh).
Have you checked if he has had any successes in the past year/5 years? If he's managing to get some deals, perhaps you should speak to him about why your 1st hasn't sold. Are you sure he's done nothing for 3 months or could he be 'talking to people'?
Anyone have any experience of changing agents? I managed to sign with the first one I tried about 5 years ago with my 1st novel but nothing ever came of it. He submitted it around but I never got a deal. I've now written something completely different and he says he likes it but has done sod all with it despite having it for 3 months. Getting fed up and thinking about submitting elsewhere but don't want to cut off my nose to spite my face etc etc. Help!