I agree get him to the vet and have him checked out.
Just an fyi however about the gas.....try a teaspoon of plain yogurt in the food. Also instead of brown rice give white rice this will help with the gas too. Mushrooms are not good for dogs and can be quite toxic. As these other foods are too:
• Chocolate & Caffeine
• Grapes & Raisins
• Pits of most fruits
• Macadamia nuts
• Products sweetened with Xylitol
I'm not sure about the celery or peas I've never heard of anyone feeding it before to dogs. Carrots do contain alot of sugar. Are you grinding up the veggies or just cooking them to mush? Alot of dogs can't handle just raw whole veggies they can however stomach them easier if they are run thru a food processor.
Keep us posted as to what your vet says. I hope your baby feels better soon.