You should have weaned him to adult food when he was still a pup. Our giant breeds to NOT need puppy food.
What is done is done. Do not sweat it. Just do it right from here forward.
Do it now, but do it slowly (like over at least a week, if possible).
Try to find something that is grain free and is reasonably close to a 1:1 ratio for calcium and phosphorus. I like the maroon bag of "Kirklands" brand at Costco for a real good 'bang for the buck' food. If he's allergic to chicken, there are other Kirklands bags to look at using.