Maybe someone can help

[motherteresa3x]

My son will be 4 in August and I can't get him to poop on the toilette. No matter what I try he just won't do it. My first two kid's are girl's and I never had this problem with them. By the time they reached 4 they were both fully potty trained except for night time. My youngest is even potty training now she will be 2 in April. She just walked up to a potty one day pulled off her diaper and sat down to pee. But she is noticing her brother still doesn't poop on the toilette and has started mimicking him.  If anyone has any idea how I can convince him to start pooping on the toilette I would be grateful.