Oh, scary! I'm so glad he's okay! It's really okay to give the sugar for these few days like the vet says, it won't hurt him and will keep his levels from diving low.
Was it really hot there? This may have contributed. While it's a little odd to have a hypo episode for the first time at this age, it's not unheard of - especially in the summer/heat. Maybe just keep some Nutrical on hand in case you notice he hasn't eaten and it's a hot day, and you're not sure if he's had enough water. I highly doubt you're doing anything wrong - it was probably just a combo of the day, and his chemistry on that day.
What are you feeding, btw - it's not Nutro/Natural Choice is it? (causing deathly problems in many dogs)
If he's not finding his food appetizing, maybe switching foods might help him want to eat more.
