dehydration in a 6 month old

My lo has some type of bug and we went to the doctor 2 days ago. Today he's been eating a lot (he's breastfed) and not really peeing but basically every time he farts he has a little diarrhea, so I was thinking he didnt have a lot of time to get them wet anyway. But of course I looked online and I'm a little freaked out.. should I call the doctor in the morning? Do I need to take him to the ER? My niece has the same thing and she had diarrhea a few days back too. Ftm so I'm pretty much always worried and freaking out.