Hello and welcome,
He is really handsome !
I had a chi / mix - might have been terrier, he was 20 pounds.
It's hard to say how much to feed, it depends on the food you are giving him.
Different brands have different calorie content for the same weight of food.
To begin with, I'd start out with the recommendations on the food package ( or their website) for a 15 pound dog. If he's getting too much, obviously, he'll start getting chubby.
Lets see more pics