Potty training, teaching boy to hold it

I'm potty training my 2 year old and it's not going well. I've had him in underwear all day and he's gone through about 10 pairs. We tried putting him on the potty every 15 mins, and rewarding him with a treat. The problem is he doesn't know how to hold it, or know to tell me he has to go.

He pees at least every 5 mins, so he clearly doesn't get the concept of holding it. I have no idea how to teach him this. Anyone have any tips?