Omg - my kid had THE BIGGEST soft spot I have ever seen! It was like the Grand Canyon. I'm posting a few pictures for you to look at!
She was born with a pretty large head, but her soft spot wasn't really noticeable until her skull plates started to spread out - and by God, did they spread out!
It was like a massive great diamond on the top of her head - you could see all the sutures and the shape of each piece of bone and everything. It didn't help that she had no hair to speak of. It was constantly sunken in when she was upright. I worried about it all the time, but nobody ever mentioned it to us, and she always fed plenty, and peed, and pooped.
Basically, no amount of fluid was EVER going to plump that f*cker out!
She still has a large head, and her soft spot is still open (she's 14.5 months), but it's soooo much smaller and her head is a normal, beautiful shape. I noticed one day suddenly at around 9-10 months that her soft spot wasn't nearly as big as it had been. She's so, so bright and clever and active too - I find it unlikely that she has suffered any adverse effects from having a sunken fontanelle for so long.
The first three pictures are of her from around 4-6 months and the last is of her a couple of months back (please excuse my stupid face in the picture - it was the only one I could find that showed how much it has closed lol). It's even smaller now - so much so that you wouldn't notice it on a photo, and it never looks sunken in.
She still has no hair though.
Watch his pee/poop output and don't worry too much about the soft spot - if it's big, it's surely going to look sunken!