Can I be rude to a person that gave me unsolicited weight loss advice?
Is this person close to you?
Yes, I grew up with him and we spend a lot of time together, but he doesn't see that I'm gaining more and more weight and he keeps bothering me with his advice
You two have been close and he is clearly trying to help you so that you won't encounter health problems in the future. If you don't heed his advice, you may end up suffering health problems and end up with one less friend too.
I am not concerned about health problems. I just want him to stop being an asshole.
I understand you seem frustrated but when people are trying to help it is a bad idea to be rude to them. We all need help sometimes so you should be more receptive.