I can't see how a neuter could cause that. I'd tend to agree with the other posters with a possibility of bladder crystals or stones. Also, back problems, like a pinched nerve or spasmed muscles, could have some effect.

A visit to a new vet that will do a urine test to see what that comes up with would be my reccomendation. He could have more than one issue, none of which are related to the neuter.