Walter McKelvie Jr., 43, was indicted Tuesday and charged with one count of child abuse and neglect. Authorities were alerted by the boy's mother, who has custody of the child but was not with him at the beach, prosecutor Meghan Hoerner said.
She took the boy to the hospital, then reported the July 20 incident to authorities. The son, identified only as R.M., suffered large, bleeding blisters on his back and face.
McKelvie told investigators he put sunblock on the boy, but the indictment said he failed "to apply enough sunscreen causing severe sunburn to R.M." while he had custody of him.
If convicted, McKelvie could face up to 18 months in prison.
A call to McKelvie's home was not immediately returned Wednesday.