Welcome to the gastroenterology community! I would recommend taking him to a doctor soon before you start any treatment. Both yellow and red stool indicate problems and it is best to get these checked out ASAP. If it turns out his only problem is constipation I would not recommend using senna. I would first recommend increasing his fiber intake, his intake of fruits and vegetables, his intake of water, and more exercise. If this doesn't work then you could start him on a mild laxative like Miralax.
have you read anything on cure zone? i think i've seen things like that there. worth a try but definitely take to doctor. i recently tried 'liquid' senna sold in drug stores; it tastes good and might be easy for a young one to take to help with the constipation.