12 mo son has yellow tongue. Will cleaning tongue fix it? Tried scraping tongue but he gags + then keeps mouth tight shut. Any recommendations?

Ignore it. It may simply reflect his enjoyment of yellow vegetables or tiny food particles that adhere to the rough surface of the tongue.
Normal or abnormal. The tongue can indicate your systemic health if a trained doctor examines carefully to find subtle signs. Consult a doctor with training in eastern medicine also. The tongue may show lack of oxygen (airway space), or vitamin deficiency (diet), kidney or GI functions, etc. Then your son can then be directed to a specialist for better health for years to come!