Usually around 6 months , they do have other indications also like lethargy,irritability,high pitched cry, trembling of limbs, seizures, eye movements ..feeding poorly ..I think if the childs parents are concerned they should take him to be evaluated, it could also be developmental delay ..good luck
Children can be diagnosed at whatever age the doctor finally makes the determination. Lots of children have your concern and they do not have CP. My suggestion is to get help before his movement patterns become ingrained.
There are Early Intervention programs in most states. They are either free or low cost. This is the number for your county
Service Area 9
Clackamas County 503-675-4097
Most children do very well with therapy supports. I worked in Early Intervention in my state for over 20 years and was always delighted when a child made great strides and it happened many many times.
I hope this helps!