My mom bottle raised a tiny little Yorkie many years ago whose Fontanel never closed. Kelsey is about 10-12 years old now and has never had any health problems. She only weighs about 2 1/2-3 lbs now, has every one of her teeth, and enjoys life without any problems. I don't know if she is the norm or an exception but I do know my mom saved her life by bottle raising her when her mom rejected her and Kelsey has always done great and never had any issues with her open fontanel. I hope your little guy does as well as she has all these years.
Teresa