My son is 14 months and have the same problem. We went to the doctor last week and found that his pa pa (where ng ng passes) is split open both up and down. The doctor explained that his ng ng is too hard and too big and therefore each time he has to force the ng ng out and hurt himself. He gave some medicine to soften the ng ng.
He also advice more water, use fresh milk, more fruit etc. I think anything to soften the ng ng would be good.