Well my ex girlfriend's four year old lad who lives with us (long story) is a very unusual wee mite and I see absolutely nothing wrong with that, he's merely his own man and yes, he does demonstrate odd and unusual behaviour ( like his obsessive love of cogs, insisting on having 3 knives and forks, and lying his clothes in a human shape on the floor, speaking only through my 8 year old daughter unless he absolutely has to) but does that necessarily mean he's "autistic" or anything else with a label attached to it? I don't necessarily think so. I've had a lot of people say we shouldn't "indulge" his unusual behaviour, but why not? He's perfectly intelligent,in fact more so than lots of kids his age, and can interact when he so chooses and yes slowly he is becoming more "normal" ( hate that word) so maybe your lad too is growing out of simple eccentricity which I believe applies to our lad, so I wouldn't panic too much yet.
Lonnie has an autistic daughter and is a mine of info on this subject so with luck he'll be along to advise you, but for myself I think sometimes we're very keen to label people prematurely.