Congrats on your new addition! It is very common for adult Chi's to have an open fontanel (molera) - as a matter of fact I believe it states it in the breed standard in the US!

Four of my Chi's have a small molera & are perfectly healthy. I guess it depends on the size of the molera as well...all of mine are relatively small. (smaller than a dime)
I can't wait to see pics of your baby. Kudos to you for rescuing him!