My best advice is to keep trying different foods for rewards. If your dog can't sustain interest in commercially prepared treats, start trying home made foods. I know a show dog that loves popcorn, and that is what he is baited with in the ring.
My 6 year old GSD is not overly food motivated. It took many, many months of trying different things before I discovered that he liked home prepared, baked liver. I only used it for training, and even then I would mix it up with a toy reward (his favorite tug toy, which he also only got when training). He never knew what he was going to get, the food reward or the toy reward.
If you can't get focus or engagement off leash at this point, by all means, keep the dog on leash.
Last edited by sit,stay; 11-22-2012 at 03:55 PM.