Lots of ways to go at this but maybe use smaller dummies than the standard ones. Mostly you might try trading a retrieve (initially toys) for a piece of kibble or, better yet, a 1/4 - 1/2" cube of cheese. Transition to dummies after he demonstrates (and knows) that bringing back whatever you throw will get him a reward. Only do it a few times in a row and then only a few times per day. Once ingrained you can remove the reward.
Parenthetically, I say "yes" whenever the dog starts back with the toy or dummy and then I produce the treat immediately. The dog learns that "yes" means he's going to get a treat and this becomes helpful whenever you're rewarding for a job well done. The "yes" marks the exact behavior you want the dog to remember as to what produced the reward. Waiting too long to say "yes" gets you all srots of crazy behaviors. If the dog "whoas" at a distance you say "yes" as soon as he stops even though the reward can come a minute later. Best to look at MIchael Ellis's introductory remarks to reward based training on the web.
