When my son is teething, his appetite is bad as well. He would reject milk and food and drink only when he is desperate and hungry.
Most importantly is to keep him well-hydrated. Once the teeth comes thru (which for my son's case is between 1-2 weeks), his appetite will bounce back and he would eat much more. Kinda like to make up for the days he hasn't been eating.
Pasty poo is ok. Loose stools can be alright if it is less than 5 times a day and not made of more water than solid bits.
Teething and appetite loss is a pattern you'd need to observe and notice.
If the poo is getting more watery and causing his bum to be red and sore, it could be diarrhea. It could be due to dirty food or a stomach bug. Then it'd be best to see a doctor because he would be in some pain as well.