Teaching abstract maths concepts to young children.

I’d not set out to teach him anything; I’d make accessible mathematics available to him and let him choose what interests him. Enzensberger’s The Number Devil introduces a wide variety of interesting mathematical ideas in a very accessible way. If (or when) his reading is up to it, Martin Gardner’s collections of columns from Scientific American are good.

The main point is that it should be up to him.

There’s all manner of accessible mathematics that might prove to be more interesting or more fun: Fibonacci numbers and their patterns come to mind immediately. Representation in other bases can be fun early on; I especially like binary (as the system that arises naturally when you want the most efficient set of counterweights for an equal-arm balance when the object being weighed and the weights must go in opposite pans) and balanced ternary (as the system that arises naturally when the weights may also be placed in the same pan as the object).

Who cares what your elder brother says? Try it and see if your younger brother enjoys it. If he doesn't, try something else. You're not going to do him any damage, so if he seems to be interested, why is there any question?