Cat meowing all the time

I have a cat that is 2 years old and meows all the time. Almost all day and
pretty much all night. We walk him reguarly and we have tried spraying him
with water and yelling at him, taking him off the window sill, putting him in
a different room. Nothing is working. I read somewhere that we should ignore
it and reward him when he is being quiet. Is there anything else? Is there
some way to keep him entertained? We have another cat and he is just fine.
Any suggestions?