The just isn't a meaningful average or growth chart for boxers. At 8 weeks, some pups are just 6 or 7 lbs, others might be 15lb or more. And that differential continues for quite some time (it is no indication at all of the size they will end as adults though).
17 lb at 14 weeks sounds perfectly fine. Actual weight isn't that important, but condition is. And on a boxer (including a puppy) you should always be able to see the outline of ribs. Not protruding ribs, but outline. It sounds as though that's pretty much what you're seeing on your pup, therefore he is a correct weight for his size.
As for how often you're feeding him though - twice a day may be a bit rough on a baby that young. Puppies, like human infants, don't regulate their blood sugar very well if they have to go a long time between feedings, and can even end with hypoglycaemia. Better to do as nature intended - feed little and often

Most pups of that age would be getting 3-4 small meals daily. And dropped back to twice daily feedings from around the 5-6 month mark.