If you have medically ruled out cystitis (bladder infection) then the cat is spraying due to some type of stress. This could be seeing other cats as the other poster related, intercat issues, new occurrences, etc. You may have to temporarily separate him, or find out what is stressing him, such as setting up a video camera to film and see what is going on. This would give you the best insgiht.
My own cat was urinating like this - would come into the living room and urinate in front of me. My female was urinating ON me! I ended up, after trying everything, putting my cat on prozac. I give him his meds once daily and the cost is about $4/month at Walmart. Would not hurt asking your vet to script this out.
I give 5 mg once daily. He stopped his urianting behavior on day 2.
Let me know if you have any more questions.