Hi, what a difficult time you've been having! I hope your boy is doing better now.
I'm in the UK where no-one would think to circ for something like this. I know one adult male who was recently circ'd after he developed persistent phimosis (after 7 months of agony and unsuccessful treatments). Sadly he hates his new state but says it is somewhat better than the pain he was in...anyway, he is nearly 40, not 5!
I was wondering if you sorted out the constipation? The wet undies made me think of a not-properly empty bladder, with the last few drops sort of leaking down, his "resting" his penis on the waistband could also cause a few drops to be held back. Does he "shake" after he pees? Getting him to use his fly or pull his trousers and pants right down out of the way while he pees and give a good shake then dry (the few drops hanging on the end of the foreskin) gently with tp might help him leak less - a few drops coming after the main even is very common because the muscles which open or close to release or hold back the pee are at the base of the penis (as i'm sure you know) so you can end up with the penis having some still hanging around in there. But if he's constipated it's possible his bladder isn't emptying well at all.
This hasn't been mentioned here yet, and possibly there's some reason which i am unaware of, but my DD got very constipated and had some UTI-type soreness when she was 2. She got into a cycle of it hurting to poop, so she would hold it back for 3 or 4 days until she couldn't stop it anymore and then cry for 60-90mins before eventually having a poo which was always rock hard and several times she got anal fissures. My poor baby!
Anyway i went with syrup-of-figs with senna. I gave her half the recommended dose for a kid her age every day for a week, then every other day for a week, then every third day for a week. She has been fine ever since. I know with senna some people fear the bowel will come to rely on it, but that really wasn't my experience, once we had remedied the initial problem and made going to poop comfortable long enough that she no longer feared it she was fine. I've used it twice since, both times when she's had a few days of not drinking enough then a fissure from a hard poop, so she can have a looser day while it heals a little. Anyway, just a thought. I hope your little guy is doing much better soon!