My little boy had autism and is about to be 4. I agree with PPs who say that autistic kids all have different interests but there are still things which are likely to be attractive, for instance sensory lights. Definitely speak to his parents though, who will know what might be a hit or a miss. Also depends on his level of development and function.
I have just bought this for my son's birthday. I checked that is was working before wrapping it (broken presents are a no-no!) and I think he will love it.
www.argos.co.uk/m/static/Product/partNumber/3557684.htm
He would also love a lava lamp but the one I already own does get pretty hot and the glass bit just rests in the base and would be easy to knock over. So maybe when he is older.
Other things my son loves is numbers, so we have these for the bath:
www.tobar.co.uk/counting-rubber-ducks
Also a giant calculator - he doesn't do sums with it but likes making the numbers appear. And this: pretty basic but he likes rearranging the numbers into two-and three digit numbers, it gets played with daily:
www.amazon.co.uk/See-Inside-Numbers-Wooden-Puzzle/dp/B000GKXY7U
He is very into Thomas, so wooden trains are always popular - parents will know what character is the favourite.
My boy doesn't like noisy things but does like music so is very keen on his electronic keyboard and also the ELC karaoke machine. He plays with them with the volume very low a blessed relief since he plays Christmas songs all year round so things that make sound might not be bad, if the volume can be controlled? He also likes audio books of favourite stories.
He also had this in his stocking and really loves to watch it - so simple but hypnotic:
www.amazon.co.uk/ABA-80094-Walking-Penguin-White/dp/B00DECNJG8
Price seems to be very volatile - I paid about £8 for it. There are similar toys - a woodpecker going down a wire pole, for instance, that are also good.
I don't know whether your cousin's boy watches TV. Again this will depend on development but if he doesn't like things that are too noisy or fast-moving, I can heartily recommend the Kipper cartoons (based on the books by Mick inkpen). Very charming and easy to follow,,but not just simple rubbish:
www.amazon.co.uk/Kipper-Classic-Collection-Martin-Clunes/dp/B0000X7SGG
Not sure whether this is any help as all children are different but these have all gone down well here