Book store, computer company, game show,..... Pay attention on what he like the most. Especially, he stay for a very long time and touch, talk or paly the thing for the longest time.
Do not forget to see the price!
If you can afford it, memorized it.
Then buy it down by youself and make him a supprise!
What about cook him a nice meal? Give him some sweet memory?
Alternatively you can knit him a sweater? It will be very memorable too.
You can go and get the introduction of how to knit from most library. Knitting patterns for sweater can also be purchased in the woollen shop. e.g Causeway Bay