So, it has been rather warm over here in the west, and I've been vigilant with making sure DS is well hydrated. I know that the a sunken fontanelle is a sign of dehydration, but how sunken?

DS is feeding much the same really, but with an extra night feed. He's his normal self, happy, alert etc. However his nappies, whilst still wet, are no way near as heavy as usual. Mind you, he has been pooping more frequently (gone from 2 times a week to 3+ times a day). His fontanelle seems constantly sunken as well. Does this seem ok or what? I am offering a feed every hour but he doesn't always want it so I'm assuming all is good?