Need advice have a 6 month old son doctor said his head is growing to fast an its off the percentile chart any thoughts please? Its over 100%

And the workup? The more common reason for this is one or both parents have a big head. I routinely measure parents as a first step. If the soft spot is still open, an ultrasound of the head can sometimes give enough information to rule out a fluid buildup, but a CT or MRI give good data. If neither study finds a treatable problem, I monitor the issue.