Is this your DS8 (not sure if you have other children)? If so, assuming he doesn't have specific fine motor problems, I would suggest that the smaller ones are not likely to be a problem to handle and they have the advantage that supplementary sets tend to be cheaper and big molecules take up less space than with the bigger sets (our DS likes to build polymers!). We bought DS-then-5 this Orbit set:
http://www.cochranes.co.uk/show_product.asp?id=398&catid=43(given that he was obsessed with chemistry *and* used to choose modelling straws as his craft activity the whole time, it took us an embarrassingly long time to twig that he needed a set!)
It's a 10mm set but actually I think the 6mm ("Minit") set would have been fine for him too; he never had any trouble handling it.
We've needed to buy specific extras several times, and this range has a lot of them:
http://www.cochranes.co.uk/show_category.asp?id=82- a bag of 100 phosphoruses makes a wonderful stocking present we found :-)
I can also thoroughly recommend this chemistry book:
http://www.amazon.co.uk/Illustrated-Dictionary-Chemistry-Usborne-Dictionaries/dp/0746077467if you're in the market for something approachable but thorough.
