I would try a raw diet if you can afford it.
This would take all byproducts out of his meals.
We have a Great Dane that is on raw and he does amazing.
If you want to stick to kibble look at the formula he's on now. Is it chicken and rice or? Try using something that is like lamb or fish.
natures choice or blue buffalo there's a few others , I go with blue wilderness LG breed for the protein but they're the best foods, DONT GIVE him royal canin they claim to be healthy but read the ingredients on the bag full of by products and chemically enhanced vitamins. though I don't think that's gonna solve your issue ... what's your yard like ?
This link will help you review foods from best to worst:
http://www.dogfoodadvisor.com/