Puppy Diarrhea??!!(PLEASE HELP)?

I have a 4month old 50lb old english sheepdog I walk him about 2 or 3 times a day he gets fed 2 cups in the morning abt 2 1/2 cups for dinner I'm not sure what kinda it is but we just switched puppy foods and I tell my dad not to but he always spreads peanutbutter around his dog bones and lets him lick it off and even gave him a little adult dog bone treat and now he's having bad diarrhea so was it the pb that caused it????!!!! and also how can I cure this??????