i read about the fontanelles as well, and when i asked a pediatrician about it, he told me that it is quite normal for the fontanelles to be sometimes sunken in, as long as baby is alert, drinking well, and having enough wet diapers.
My girl used to have fontanelles that sunk in a little when she was younger, even though she fed well. I just monitored her pee, and made sure i offered her breast milk whenever she feels like it.
Nowadays, her fontanelles dont't sink so much.
If your baby is 6 months and above and having solids, you can feed him/her water whenever the weather is extra hot, or when you notice that baby is perspiring a lot.