You may be over using the treats. Try it without the treat, or hold the treat until you release the dog with OK or free dog. Make eye contact with the dog, and call its name and ''down!'' it a quiet, but firm voice. Hold your hand out, palm down. Do not repeat the command, just hold eye contact. Gently pull him down by the forearms if you have to. Praise him once down. Eye contact is very important.
You may have to go to calming and settling him first. Likely he is big enough now to use placing him on the floor on his side with his legs away from you. use your hands on his neck shoulder area and middle to hold him in that position. When he is quiet, praise him. When he accepts that position well, handle his paws and muzzle, while keeping him quiet.
The settle is like a down stay, but not the same. It must be done on leash. encourage him onto a down. he can be on his side or belly. Anchor the leash with your foot so the puppy can't get up. If he is quiet, praise him. If he struggles, keep him anchored until he stops struggling. Stay calm. At least wait for a brief pause, and then release him. Repeat until he will stay quiet for 30-60 seconds. All these may work better when the puppy is tired.
You are dealing with a big boy, and perhaps a difficult one at that. Some dogs are just more of a challenge than others. I think you would do well to pick up a copy of The art of Raising a Puppy by the Monks of New Skete,
http://www.dogsbestfriend.com/ I am sure there are many other excellent books out there, but I know the Monks' books are great.
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