When I feed my boas I try to make sure you can tell they just ate if the snakes are not full grown yet. The next step up from a hopper would be a small adult or an adult.

Just remember, when you are looking at mice sitting down or walking around they look a lot more bunched up and fat than they will be when they are going down your snake.

Why won't your dad let you buy a feeder rat? Does he realize that one day mice will not be enough for your Boa? Tell him that it could be good to go ahead and start the snake on rats now so that you won't have to switch his food items later on.