My 6 year old has the dark ring all the way around his neck he is over 80 pounds he does go outside and play and run around what could this be?

Rash: Dark ring may be acanthosis nigricans especially if a person is over weight. Acanthosis nigricans represents risk factor for diabetes. Other causes like dermatitis, hyperpigmentation from sun exposure, fungal infection are a possibility. Need to see child's PCP to determine the exact cause

Acanthosis: Without actually seeing the area my best guess, given the limited information provided is this could be acanthosis nigricans.Acanthosis nigricans is a brown to black, poorly defined, velvety hyperpigmentation of the skin.It is usually found in body folds,]such as the posterior and lateral folds of the neck,the armpits,groin,navel,forehead, and other areas. Follow-up with peds doc for evaluation