my son needs help but won't take mine

Hi, I have exactly the same problem with my son. His face is oily and covered with pimples, but he is in puberty, which makes him too cool to talk to me about it. He won’t take my help, because he thinks he knows everything best. However, he adores his older sister, so I asked her to help him with this acne problem. She didn’t have to ask him twice, he was delighted to except her help. I bought proactive cream in pharmacy, and my daughter gave it to him. He was using it regularly, and after couple of weeks, the difference was obvious. I know this period will go away, he will realize in couple of years that I’m not bad and stupid just because I’m his parent and authority.