Hi,
I just would say, I usually think about oppsite side, how
shortness or tightness antagonistic muscles, so test for a suboccipitals.The deep cervical flexors make a small range,cranio-vertebral flexion,rather than over use SCM,or scalenes make a cervical flexion.
First, check his/her posture(cervical,thorasic spine etc..)analysis. check side view, if his/her head is forward from plam line.or he/she has a huge kyphotic thoasis, maybe,his/her deep cervical flexors are weak or shorten of the suboccipitals. or caused by kyphosis...

sorry poor english advise..
good luck.