Wangthang,
As a parent of young men who do PE, I would suggest that he wait a couple more years before he starts. Boys typically don’t quit growing until around 25 or thereabouts. There is no reason to think that he might not gain a bit more just naturally due to growth process.
At this age, boys have a mixed view of themselves as it is (so do girls) and to give the message to your son that he is somehow inadequate in the penis department probably isn’t a good idea. If he asks you about it, tell him the truth and let him decide for himself, but I would not push the idea on to him. Your post was a bit confusing here, is your son jelqing and stretching, or are you the one who has been doing this and has seen gains?
And desol is correct in that your son cannot join the forum until he is 18, by then he should have enough information to make a judgement for himself. Personally, I think its great that he knows about PE, open relationships of this kind are best for families and you and your son have what most don’t, communication.