Hi Ihave a 9mth old boxer won't eat for days on end ribs are showing. Please HELP

Take him to the vet. There's nothing we can do.
How many days exactly? Does he get into things? Anything he could have eaten?

Has he been checked out by a vet to rule out any medical conditions?
Hopefully he is just going through the fussy stage but the only way you will know for sure is with a vet visit, and until then we cannot advise you of any diets or strategies in case it goes against future treatments.

Has he been vaccinated? Is he drinking? Are there any other symptoms such as vomitting, bloody loose stools, lethargy, etc..

He could be going through a fussy stage but I would not risk it being anything much more serious. You need to schedule an appointment ASAP, but if he is also not drinking then you need to find an emergency vet and not wait even a day longer.