OK for the sake of the original poster here, lets just say it could be a cause, and leave it at that.
AJ
Alot of websites base information on what is written on peer sites and insubstantial medical advice. Based on the advice of all the genitourianry registrars I have worked with I am telling you it is rare.
Actually its not extremely rare. Its quite common.
http://www.webmd.com/sexual-conditions/guide/20061101/oral-sex-may-spread-common-std
http://www.ashastd.org/learn/learn_ngu.cfm
http://www.medhelp.org/forums/STD/messages/C260-1.html
NSU from oral sex would be extremely rare although it's an outside possibility.
NGU/NSU can also be caused by normal mouth bacteria entering the urethra during oral sex. If you perform oral sex on him, this could indeed be causing his symptoms.
When he goes to the doctor (and he should, because he'll need an antibiotic for it), his doctor should check his urine for white blood cells, along with gonorrhea and chlamydia.
AJ
Yes it could be non sex related. Men can experience what in the UK we refer to as non-sepecific urethritis (NSU), in the US I think it's called NGU (non-gonnococcal urethritis). NSU can be caused (in rare cases it has to be said) by too much masturbation or by an overgrowth in any other normal bacteria in the urethra. If he has taken antibiotics recently for instance, this can cause some bacteria to die which creates a situtaion where the ones left over can thrive and over grow. Normal bacteria that would usually be kept in check by each other can get out of hand and cause burning or a discharge. NSU normally presents with a discharge though it can occur with simply burning on urination. One other thing, it is very common for people to have some discomfort when urinating if they have not had enough water, this commonly occurs first thing in the morning of after a drinking session. Speaking of which, alcohol can cause urethral inflammation. Having said all of this, you should know the most common cause of NSU is chlamydia and another infection that cuases burning on urination is gonnorhoea. It isn't time to dump him yet but you should go and get both of you checked for STDs and while you're there the doctor will make a diagnosis of NSU or otherwise and prescribe an antibiotic to clear it up. Good luck.