I think the focus in this situation should not be on "dressing well", but specifically on making sure he understands the effect his choices have. Many people that age see choices of dress as expressing their individuality, and thus being choices that only affect them, but fail to see the effect those choices have on others, and indirectly on themselves from those effects on others.
Generally, addressing something like this with a teenager should be with an adult-level conversation, where you treat them as an adult, just as if you had a friend with body odor issues.
I would suggest talking to them first about whether they understand the effect their hygiene and dress choices have on others' perceptions of them, and what the long-term impacts will be of those choices. If they understand, or at least profess to understand, those impacts, then there's not a lot more to do; you can't bathe them anymore, after all, and it's probably better for them to learn the consequences in high school than in the workplace. But it's also possible they will tell you that they're aware of the issues but don't feel like they can fix them, in which case you have an opening to help.
Try to be specific about the particular issues, though, and not to focus on generalities like "dress your best"; and I would suggest limiting the discussion to the most extreme issues, not just wearing a polo with ripped shorts, as some of that is generational disconnect - I'm a middle-aged adult and I don't find that particularly unusual as a choice for a high school student, really. The things I see as issues are wearing clothes for multiple days in a row and not showering often enough.
My children aren't as old as yours, but my eight year old has responded well to us simply pointing out that if he doesn't change his underwear he smells bad, and that will make people not want to hang around him.
I would also consider probing a bit into whether there is an underlying mental health condition related to some of these choices, as well. Many teenagers (myself included, back in the day) have body issues, whether they be body image (external) or body comfort (internal), and getting naked to change clothes or shower can be very uncomfortable, even in their own space.
It may also be that he feels he can't make good choices in dress, so he'd rather make obviously bad ones - whether as an element of perfectionism or to avoid being criticized for small faults. Maybe he feels he doesn't have nice enough clothes to match the nicely-dressed kids at school, or maybe he lacks the fashion sense to do so; it's possible he could even appreciate the help from you. But the only way to find out is to have an adult-level conversation about the choices, and see if it leads into the "why".