Many 6 month old babies start to get teeth. This can make a baby cranky (surprise!) and drool a lot more. You can help by rubbing his gums with a cold cloth or by giving him teething toys.
Try adding a little variety while you're at it. Offer smooth toys and bumpy toys, warm toys and cold toys, soft toys and firm toys. You'll ease the pain in his gums, and he'll learn a lot about how things feel.
Teething can cause diarrhea in normal babies, so don't be alarmed. If it lasts a long time, or comes with fever, vomiting or other signs of illness, call your doctor. It could be something more serious. In time, you'll learn to read the signs that signal your baby is teething.