There are glucose tablets, chewable, that you can get at any drugstore & some grocery stores. They're flavored, too.
I carry them with me, just in case. With these tablets, all you're getting is a blast of glucose, and not the other crud. It depends on how low your sugar is, as to how many you need to chew up. He should have a monitor and when it appears that his sugar is dropping, check it, and then yiou'll know pretty much how many he needs to eat.
If it's below 40, call an ambulance.