Every day, standards of "aesthetic appeal" are becoming harder to meet. Especially for the ladies, they have to take time to put make-up on, look good for the night out, the club, or even for sorority recruitment. In this context, "aesthetic pleasure" means the following: Looks do matter.
As a result, the man's perspective of what "beauty standards" are is being overlooked. They're not called "beauty standards" necessarily, but for gentlemen, "looking good" matters just as much as it does to the ladies. Just like you should never say the following things to a lady, you should never say them to a gentleman either.
"You need to lose/gain weight."
This is more thrown around with the ladies, but a gentleman takes offense to this too. He actually tries to be fit and take care of himself, and for someone to say that to him is a painful shot aimed in his direction. He has people close to him in his ear about his weight even though he appears to be in shape, and they don't need to be. He's aware of himself and he's working on it. He'd rather hold himself accountable than for you to be a b**** to him.
Also, weight is just a number, and numbers don't lie, but they can change. It's a process and no one should be forced into doing something out of their comfort zone unless they really want to go there. Furthermore, telling someone to lose weight based on if they appear fat is calling them something that they already know. Physical appearance doesn't tell-all about someone. It can be potentially deceiving, where it's possible to appear large but have a lower weight or appear small and have a heavier weight. He appears healthy and that's already better than many people can say.
"Don't eat so little/much."
Really? Shut the front door! He can eat however much or little he wants. There's a reason that he does it and no one knows unless they've been told explicitly what it is. It's not your right to question him and unless he doesn't mind you asking, don't expect a positive reaction to such a comment if you do question it. It's judgmental to assume things about someone straight to his face based on the amount he eats. It could be an experiment, or a habit. Again, what you see doesn't tell-all.
"You look like you've lost/gained weight."
This ties in with the first one, but the same concept still exists. Saying that someone looks like they've lost or gained weight suggests that they were different although he may not have really changed that much. He is human, so it's not right to assume that because he appears to be different it makes him more or less special than he was before. The fact of the matter is, he's still the same person. The only thing that really changes from weight loss or gain is the physique, but is someone really defined by their physique? No! Absolutely not. Saying that someone looks like they've lost or gained weight might get a sighing response of "Thanks for telling me." where it's something they already noticed themselves.
Instead of saying the above things, if you see a friend who's doing something different than you're used to seeing them do or if they appear different to you, say something positive like, "You're looking sharp, man." or, "That looks delicious! Where did you get that from?" where it reminds them of something that they're doing to feel/look good or to enjoy themselves.
The main point that is supposed to be driven home here is that Isaac Newton's third law applies to the way you talk to someone about appearances / looks just as much, if not more, than it applies to physics. But the same saying rings true: Every action has an equal and opposite reaction.