My husband wants to get his tongue pierced but uses smokeless tobacco. will he get cancer at the piercing?

Unlikely: Smokeless tobacco is so bad for so many reasons. Although the whole mouth and throat is at risk, the tongue, with a piercing is not at extra increased risk.

Maybe not, BUT......: Maybe not cancer, but tongue barbells can chip or fracture front teeth. Gums can be damaged by the rubbing of barbells requiring periodontal surgery. If the barbell becomes unscrewed it could be aspirated into the lungs. Infections can be quite severe & eventually lead to disfigurement. There can be significant blood loss & permanent neuralgia (pain).

Smokeless tobacco: Regardless of the piercing, the smokeless tobacco is more of a cancer risk than almost any other behavior. Please quit.

Tobacco and cancer : Tobacco is responsible for many cancer (smokeless also).. Piercing by itself is unlikely to cause cancer but tobacco use is a big risk factor.