“What did people do before they invented hats and jackets?” “I dont like dressing that way”. etc “
Unfortunately there IS no real answer to that that will ever satisfy the child who does not feel connected to this type of levush. There was a time when the frummest of the frum wore turbans or long robes (no sox) or fishermen’s caps amd knickers, and CERTAINLY no Borsalinos or suit jackets. That I can promise with no doubt. Many of our young people feel the stress put on the levush is hypocritical, especially when they hear of “frum-looking” people committing financial fraud.
I agree that too much emphasis is often put on this, to the point that a young man who does not follow the dress code is deemed almost irreligious by foolish people. However, the issue you are describing is partly the natural rebellion of a child and partly a reaction to some other feeling of negativity associated with this mode of dress or to the formal rituals of Judaism. The more you push, the more he will push back. I think the best advice is to speak to someone (NOT necessarily your own Rov), who is experienced in dealing with kids who have off-the-derech issues (NOT that I believe that the failure to wear a hat and jacket signifies being off OTD, just that it indicates a rebellion against the outward religious accoutrements of your circle). Have you tried ascertaining what his objections are? Can it be that he dislikes his rebbie or some other aspect of school, and is reacting to that by “blaming” the hat and jacket (as symbols of his dislike)? Or is it simply that he does not feel the need to wear these things in order to be frum? If so, and if you live in a Yeshivish world, it is obviously going to be more problematic than if you are in a neighborhood where frum kids come in all manner of (tzniusdik) dress.
Obviously this means a great deal to you, but I would tread cautiously. You don’t want to see your son turn this into a snowball effect and alienate him altogether. Talk to him, and mroe improtant, LISTEN and hear what he is saying.