I personally don't like Science Diet. It's full of cheap fillers like corn and dangerous preservatives like these:
Ethoxyquin
Promoted kidney carcinogenesis.
Significantly increased incidence of stomach tumors.
Enhanced bladder carcinogenesis.
BHA
Enhanced stomach and urinary bladder carcinogenesis.
Causes squamous-cell carcinomas in stomachs.
(Cancers of this type are among the most lethal and fastest acting, the swiftest effects being seen among animals with light colored fur.)
BHT
Promoted urinary bladder carcinogenesis.
Could be a promoter of thyroid carcinogenesis.
Studies have noted that BHA and other antioxidants, particularly Propyl Gallate and ethoxyquin, showed additional effects in inducing stomach hyperplasia and cytotoxicity. http://www.canine-epilepsy-guardian-...ogy_of_bha.htm
I doubt you can blame the food on his weight, though. I think he is just genetically programmed to be a big boy.