An open fontanel is the soft spot on the top of a puppies head. Congenital open fontanels are often found in toy breeds.The fontanel is the area at the top of the skull where the bone plates merge. This will usually close when the puppy is 4-6 weeks of age, but sometimes they never completely fuse, leaving a hole in the top of the skull. If it has not closed by 12 weeks of age, it probably won't.Extra care should be taken with a puppy with an open fontanel. The dog should be spayed or neutered as soon as it's old enough so that the genetic trait won't be passed down the line.Some dogs with this condition suffer from hydrocephalus. He might also suffer from seizures, vision problems and uncoordination. He also might not ever have problems and live a long and happy life.
You need to see your vet to have him examined as soon as you can.
Let me know if you have more questions.