I don't think he understands the meaning of "chi"線. He would just copy this from people around him.
Try keep on repeating to him that he should say this and on the other hand teach him to say other words such as "You are nice, I am a good boy, I am happy" etc whenever you hear that so as to district him from repeating the bad words.
Or if possible (if I were you) I will try minimise the chances for him to meet with relatives and friends that has bad attitude and speak impolitly.
其實佢地呢個年紀, 好多野都學, whatever good or bad, It is so easy to understand why our children say these, So If it is not really un-accepted 我會話 “你講曳野”. Take if easy, Sometime, you will find our children talk less about it and then talk softy on these and then do not say again