My son is 17 and when he walks he has out toeing. this started about 3 years ago. his knees turn inward now. what would cause this?

Flexible joints.: People with very flexible or loose joints may tend to have flat feet or tend to walk more on the instep and roll the heel outward. This changes the alignment of the leg, so the knees lean in as well and is more commonly seen after puberty growth and also in children with low muscle tone. Being overweight may also aggravate the symptoms. Blounts disease causes bowing. Shoe orthotics may help.

Bow Legs/Knock Knees: Occasionally, children have problems that result in abnormal alignment of leg bones, some of whom may relate to serious injuries, metabolism or nutrition, or other orthopaedic problems such as Rickets or Blounts disease. Bowleg can be caused by Rickets, Blount's Disease, trauma, a fracture healing incorrectly.Bowleg, when not part of normal development or trauma, is a symptom and not a disease

I agree. Also, see : his pediatrician for evaluation to determine the cause of his change in gait. Document any symptoms of pain, limping, limited mobility, etc. Depending on the history & physical findings, (s)he may refer him to a pediatric orthopedist, physical therapist. Orthotics, inserts for shoes may help correct pronation of the ankles.