I don't know what's happened ... but whenever he plays on the XBox driving games he now puts on a very good 'American' accent. It is frighteningly good in fact !
The only thing we can think of is that he's picked it up from his cousin who was over a few weeks ago and they played on it together quite a bit (Liam get's on really well with him even though he's about 5 years older).
On reflection I guess it is a bit more Canadian than American so I am not as worried about it () ... it's really strange to hear a kid so small putting on an accent though !