This dog sounds like he needs to go on the BARF diet - you will find plenty of advice on this site about feeding raw food. Dogs are not designed to eat large amounts of carbohydrate, and one of the side effects of doing so can be the symptoms that your newfie is suffering from.

2 or 3 weeks of raw meat, vegs and meaty bones will see him as right as rain in no time.

If you google "Barf" you will find loads of interesting sites to help you.

Good luck!