My dog thinks my son is a small animal

Help! How do I get my dog to realize my son is a human ???

My dog is a jack Russell terrier. He’s 13 and a bit high strung. I’m really nervous he thinks my son is just a toy.

He tries to scratch at him and nip him not in a mean way but... in the way dogs play with toys.

How can I help him understand my son is a human not a toy? Please this is so stressful.