Most adults still haven't figured this out, so cut him some slack :)
The fact that he blew all his cash on one toy, and now realizes that he has to work for more WAS the lesson wasn't it? Better to learn this with small sums now than later in life with big ones!
I cringe when my kids spend their money on crappy dollar store toys, only to have them break almost immediately. It took a couple of years, but now at ages 6 & 8 it is starting to compute. My 6 year old still wants everything he sees, and I keep having to remind him his spending money is gone and his saving money isn't for this purpose. My 8 year old is now saving for better quality toys at the other store. Baby steps.