He's probably dismissing you because first, men don't get UTIs except in three rare circumstances. And second, if you're unwilling to do an STD screening, you're crazy. Besides that, you can get FREE screenings in just about any city. And third, your doctor would be an irresponsible idiot for prescribing antibiotics without confirming the type of infection and which antibiotic is appropriate for it.
You should have asked him (and still can), what he was testing. It's possible he was doing a quick test for gram positive bacterial infections, but it's your job to get your questions answered before you leave your doctor. Call the office and ask what test he did. It will be on your chart. He could have been looking for proteins and sugars.
Dark frothy urine can be symptomatic of many things. Yes, drinking more water will help, half a gallon per day. Froth is usually suspect of protein in your urine, which can be a problem, but there are also some diet choices that can also affect color, smell, and appearance of urine.
You shouldn't be waiting for your bladder to get full, that's usually too long. If you have a regular schedule, figure out the interval that's appropriate, and just go whether you feel urgency or not.
Anyway, I'm not sure what made you think you might have an infection in the first place. UTI symptoms are highly unpleasant and rather unmistakable. By the time a male has symptoms, it's an emergency situation.
As far as your urgency without actually needing to urinate, this could also be an allergic reaction to something you are eating or drinking (which could also explain the dark urine). It could be related to a parasitic infection like from shellfish. Or it could be related to anxiety or life-stress.
When you drink cider, you're influencing the pH of your body, and introducing phenolic compounds that are often useful in digestive processes. Plus if it's real cider, it's fermented so it's alcoholic. So, lots of reasons this would change your urine's appearance.
Go get a free STD screening. If you're sexually active and haven't done so in the last year, it's time to anyway. There are lots of free clinics, and many that do it for less than $40. If you've been taking antibiotics at all, you should start taking some probiotics for at least a month to repair the damage antibiotics do to your immune system. And by all means, call your doctor's office and find out what test was done.