My problem is that I don't know a lot of "smart" things. A normal offline toy shop does not have much to offer. And you have to know what to look for online.
When I was a kid I had a number of Lego Technic sets - they are great! Might not be proper "engineering" but still allows you to touch and build a simplified version of many real-world things, for example a car's drive shaft.
If he's already pretty skilled, he'll definitely be able to handle even the sets that are marked 10 years old +. I recall I had one of these when I was 6, it was a slight challenge but that was the fun part : ).
Yes, he handles 9+ pretty well. He's now playing a lot with Fischertechnik (http://www.fischertechnik.de/home/produkte.aspx), it's a German analog of Lego Technic (or vice versa, whatever), but as far as I can tell it's somewhat closer to reality. I'll take a closer look at Lego Technic though, thank you.
Oh, yes, Meccano is a good hint. Requires different mechanical/fine motoric skills compared to Lego Technik/Fischertechnik.
As for Infento - looks very neat, a bit too "prepared" for my taste. We do such things from wood and stuff you can buy from the local hardware store.
Here's an example of something we've built in a day:
Get in touch with a local Hackerspace. There are lots of them in Germany, usually related to the Chaos Computer Club. They'll happily provide you with pointers and directions about what to get and where to look for it.
http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3D...
If he's already pretty skilled, he'll definitely be able to handle even the sets that are marked 10 years old +. I recall I had one of these when I was 6, it was a slight challenge but that was the fun part : ).