The balls are producing the testosteron and also the hormones that make you horny. As far as I know, a man without balls looses his sex drive (thats why they ********* the eunuches).
But the cock is just used for rubbing and cumming. When its gone, the guy is still as horny as before

so if you are lucky enough to get a guy with balls but without cock, you will have the most horny guy on the world in your hands