Because he discovers that he is going to be returned to his original white family which he is not happy about. He already considered his Indian family as his.
Explanation:
True Son in the story "The light in the forest" by Conrad Ritcher was adopted by Cuyloga and Lenni Lenape When he was four in the Indian village of Tuscarawas after the couple lost their child to "yellow vomit." They treated True son as their own child and True Son does not consider himself white anymore. Therefore he was gloomy when he learnt that the indians are being forced to return their white prisoners. He decides to blacken his face so that He could stay with his Indian family. He loves his Indian family and refuses to live with the whites. but his plans failed as Cuyloga his Indian father ties him up in the village and led him out while the rest of the family watched on.
