I appreciate your concern with your baby and the diarrhea and the diaper rash. Yes, the diarrhea may be caused by teething. I'd say make sure he's getting enough fluids and pedialyte is probably a good choice.
As for the diaper rash, what soap do you use to bathe the baby? Soaps can be very harsh and can damage the skin, so may be a reason for the diaper rash. Use a mild cleansing bar. If you use disposable diapers, be sure to change them often, before they get soaked with urine. Or if you use cloth diapers, your detergent might be too harsh. Many mothers recommend Shaklee products which are vegetable based and chemical free. For more information on these products go to www.giftsfromnature.com.