I was thinking maybe lego (too difficult?), Duplo (too easy?), K-nex (don't really know what this is, but have seen it in the shops)...basically something along those lines that he can build loads of different things from. They have something at his playgroup that is made of of loads of cubes and long bits that you click together and he spends hours playing with it, but I don't know what it's called.
I know the sort of thing you mean, my DS used to play with a similar thing at nursery. Duplo probably would be a bit easy, K-nex IME is quite hard until they are 7/8 (I found some of the bits quite hard to connect together). Lego probably your best bet but probably better to buy a big tub of it rather than a specific model to make.
Thanks for the suggestions. The zoob stuff looks good, I have never heard of it before and is the sort of thing `i was thinking of, so he can just use his imagination and build something that probably resembles a dogs dinner, but he thinks looks like a space rocket!
ELC also do something called big builder - I think that's what it is anyway. It's like giant plastic meccano and it's great - my nephews have a set each and build amazing things with it. Sory to be so vague adn can't even search for you as have a wriggly baby on my lap!