Oh that's so frustrating that they did no testing.
From receiving oral sex, a male can get gonorrhea, NGU, syphilis, and hsv1 genitally, and rarely, chlamydia.
Syphilis and herpes don't cause these symptoms, so we can ignore those.
Gonorrhea and NGU (which is a non-specific infection in the urethra that can be caused by many things, including normal mouth bacteria or things like adenovirus, which usually causes upper respiratory infections) could cause these symptoms, but he wouldn't have gotten symptoms that fast. No infection gives you immediate symptoms - they all have incubation periods, which is the time from infection to the time you have symptoms.
If it is an infection, he could have gotten it prior to this encounter. The doxy will cure NGU, but won't cure gonorrhea.
It could also be something non-sexually related. He could be dehydrated, have an inflamed prostate, a UTI, or a host of other things, which is why it's so problematic that he didn't do ANY testing at all.
The best he can do right now is to take the doxy, see if it clears up, and if it doesn't, wait at least 3 weeks after taking the last doxy, and find a doctor who will actually do some testing. Another problem with treating without testing is that taking antibiotics can give you false negatives on testing for a few weeks.
I'm sorry you and he may have to deal with this for a bit longer if the symptoms persist.