Keep in mind that the way someone dresses is heavily related to their identity, their personality.
When she was a teenager, my fiancée discovered a style to which she is still sticking to now. The reason for that is because when she tried it out, she realised that she was feeling much better about herself, because she could now fully express her personality.
Although I understand why you are worried about the way your son look, and the fact that I myself like to tuck my shirt, you should be aware that, ultimately, it is your son's look, not yours.
As such, I agree with the answer Arno posted about sitting down and having a conversation with your son about it, if he wants to talk about it. Keep in mind though, the point is not to argue, but to listen and share.
I don't have any child, but I would suggest you two approaches about this:
- The first approach would have you start off by giving your point of view, the reasons why you were asking him to do what you want, ask him if he understands, then ask for his personal point of view on the matter. That last point would, hopefully, allow you to understand why he did not want to tuck his shirt.
- The second approach would be like the first one, but instead of asking for his point of view last, you could try to ask it first before giving your own point of view. This can give him the opportunity to express himself first. But then, he could also perceive it as "she is going to lecture me". You should pick the one that you think would work better.
Another point worth mentioning: an answer mentioned "social norms for attire". I must admit, I have noticed children having some kind of dress code or following a trend. Even my parents used to always tell me "this is the trend, you should do it", but I always expressed my disapproval about it, because I wanted to be myself, not fit in with the others at the expense of my well being.
In the end, you should discuss it with your son, but trying to force your point of view onto him, even if you somehow manage to convince him, is likely to have a negative impact on his moral on the long run if his own well being is repressed.
Another point I would like to emphasis is: whenever possible, you should try and look out for the reason, the intent, why your son does what he does.
Many answers/comments pointed out some interesting points about authority and the fact that the children should respect their parents, their elders, which is right. But if they happen to disobey, you should try to ask them why they don't want to do as asked before jumping to conclusion about them rebelling.
I will give you one very simple example below. I could give several, but one should be enough.
When I was around that age, there was this pair of shoes my parents wanted me to wear. I expressed several times that I did not want to wear them because my feet were feeling extremely uncomfortable - it felt like it was jamming the blood flow in my feet - especially since I had an older pair of shoes which were still very comfortable and not even worn out, so I could just take them.
They still forced me to wear them, because they believed I was just being a difficult child and I was disobeying them. Simply put, after an entire day wearing them, when I came home, it was so painful that they finally decided to believe me when I told them I just couldn't wear them.
That was just one simple example. I would like to point out that, even in my example, any random child could have been lying and acting up the discomfort too, but I wasn't. It's up to you as a parent to determine if there is enough trust between you and your child. Discipline them as needed, but I highly suggest to look out for the reasons before leaping to conclusions.
I know I'm repeating myself a lot here, but you should really, really get your child's point of view before taking any action, especially if you are under the impression you are about to assume that he's not acting on good will.
I have been on the receiving end of that with my parents, more often than I can remember. When it happened once or twice, it only made me think that it was unfair, so I tried to shrug it off, better luck next time I guess.
But when it kept happening and my point of view was forcibly overruled all the time, it left a very bitter aftertaste (and I guess a psychological scar). Because they wanted me to obey at all cost, they pushed me into getting very defensive and holding my ground against them, especially if it's about a subject I definitely know better than they do.