Charlie had this a few weeks back and his fontantalle sank REALLY quickly which totally freaked me out.

Get to a Dr or the local hospital if you are in the least bit worried.

Another option to get fluid into them in the meantime is watermelon (to suck on, or mashed, if he'll take it) or simply a clean washcloth soaked in cold water and let him suck on it time and time again......this worked for Charlie, even when he wouldn't take any water or anything else from a cup or bottle.......I also put Charlie in the bath a few times as I know he always sips water from their bath cups!

GOOD LUCK