What is wrong with my Male Cat?

So, It all started about three days ago. I saw him spray in my backpack, which he hasn't done ever since I neutered him. He is 4 years old, and I didn't take much of it and I thought to myself , "Maybe he's just being a brat".  Here is what actually bothered me, I woke up the next day with cat urine all over my bed. I was confused and thought to myself maybe he's trying to tell me something, and I took him to the vet in fear that he might have kidney stones or UTI. The results came back negative, and he said that the cat does not have any bacteria in his urine. The vet then suggested I get a culture (idk how to spell) if I want to be safe. I paid $100 for it, and am waiting for the results. He kept reassuring me that my cat was ok. This is where I started to freak out. The next morning AGAIN he peed on my bed. I further inspected the urine and saw that it was shiny, almost grainy like. I instantly called the vet and he said that " The cats pee looked healthy and hydrated", so I am so confused. Is there something wrong with my cat or not? And here is a theory that I have. I actually do not see my cat pee on my bed at night, and I have another cat that is a girl. What if it is the girl cat instead of the guy? Even so, I do not have enough funds to go to the vet again. Thanks, let me know what you guys know. 

My vet also gave me "behavorial pills" and thought that it might be his personality. but that sounds odd to me.

I have 2 males, and one of them is constantly getting in the "spraying" position and even wiggles his butt with his tail in the air, but nothing even comes out.  That very first night when you said he sprayed your backpack, did it actually get wet, or did you just see him do what I described above?   What I getting at is the same conclusion you might have come to....perhaps it IS your girl cat, not the male.   Keep a very close eye on her and see if she's displaying any signs...squatting places other than the litter box, going in and out of the box excessively, staying in the box for longer periods of time than normal.

Yes, I have heard of giving pills to cats of pee outside of the box.  I think it's something like Prozac.  Please don't give them to him until you are certain he's the culprit.  And before giving them, you might also startup a post in our Behavior Forum to see if anyone has any suggestions for you there. 

Has he been under any undue stress lately?  This can cause issues with cats.  Is your female fairly new to the household?  If so, I suppose it's possible your male is trying to mark his territory .

What's his urine pH? What are you feeding him? Get a copy of that urinalysis result and post it for us. This sounds very suspicious for a UTI or at the very least some crystals or a high pH.