
How many feeds per day are you giving him at 11 months.
By this age my cockers are ready to take just one feed per day and hold very good condition.
Never leave food around, if it has not been eaten within 15 minutes take it away completely!
Stop all treats, the dental chews are normally based on wheat, along with any fat content they add to a daily food ration. (raw bones are by far the best soloution for healthy teeth and gums! )
Could you ever consider using raw meat, many pet stores now stock this in their freezers. (Natures Menu) does fantastic frozen complete diet nuggets.