I would go with frozen... or better yet, fresh dead. Some dogs get more excited over something that still is pliable and can be somewhat "rough housed with" if it is loose. Granted, the last thing you want is a dog "shaking" the birds on the way back, but by making it fun for him, he will grow out of it... he's still young and needs some time to have fun.
One thing I did for my second shorthair when he was younger (I had FF my oldest dog, and can honestly say he's a robot, but it was something I really didn't want to do), is I had someone stand by with a clump of grass/dirt mixture. Make it look like the area you will be working in... and toss it into some grass (taller the better, but not too tall, maybe only a few inches or so higher than normal) and send the dog in. When he's in there "searching" throw in that fresh dead/frozen bird... especially if it is something he loves to pick up and bring back. Do this for a while and see if it works. Your help will have to throw the clump, you send the dog, and after a few seconds of him "searching" for whatever it was that was tossed, toss in the bird when he isn't looking.
Especially if he likes to retrieve a frozen/fresh dead bird, it will act as his reward for a very knowledgeable search and keeps his mind motivated. If this "game" doesn't work, I would say you might have no choice but to FF, sorry.
Good luck with your pup! Let us know how it goes.