Weird ! He can say seventeen but not seven

My son is extremely weird. We still have hard time teaching him how to say correctly the number one and seven. We don't know why, he can say the number seventeen correctly but still cannot say proper seven. His seven is pronounced as "Saaa". His one is "none". We kept on correcting him but he still insisted to pronounce it wrongly. He even becomes angry when we corrected him. Aiyooo. Don't know how to ....