I think the Take Along castle is really good, plus you can store the little bits in the actual castle. If he likes action figures there are lots of good stuff in that line - helicopter, safe crackers etc. The pirate stuff is good too - DS has played with his pirate ship regularly for about 2 years and still likes it.
Playmobil absolutely rocks. Ds is almost 9 and still plays with it all the time (as do his friends when they come round). The police van is cool, as are the robbers with the safe (real gold bars in it!). He loves his ranger's house, which has lots of little animals and woodpeckers and whatnot. It's not cheap but there is a lot of play to be had from it (for my ds at least - but then he's into animals in a big way). Actually he has tons of the stuff, he has been collecting it for years now. The individual figures and smaller sets are easy enough to save for with pocket money, the bigger ones are ace at Christmas/birthdays. Thinking about it I think it's hard to go wrong with Playmobil..
We have a reasonable amount: Airport Hospital Plane Camper van Cargo Ship Crane Ambulance
Pluse a lot of random bits and pieces - pony set, lions from the zoo set etc. I find the vehicles get played with the most - dd (7) will stack all the poeple round th sides of teh cargo ship and pretend they are going on holidy - ds (9.5) will also will set up scenes and play with the poeple and the vehicles. Neither plays much with either the airport or the hospital - think they may cramp their imagination actually whereas pushing aship around the living room pretending you ar etaking people on holiday is more fun!