Interesting. Number sequences consist of objects that are all of the same type, they are all on the same level of abstraction. What if you asked him to remember not only numbers themselves, but also things that are on a different level of abstraction, e.g. the structure in which they are arranged? For example, if you made him memorize a tree, you could ask him to traverse it in some order.

Or perhaps you could say a short number sequence that has some non-obvious hidden pattern and ask him to discover that pattern and tell you the next number in the sequence.