'Whizzing' on the furniture

Well .. our male Westie Piper (9 months old and neutered @ 6 months) has decided that it is perfectly OK to 'whizz' on the couch !!
Not on the leg of the couch .. on the COUCH itself!!
First time was Xmas morning .. at first we thought it was the ages old kitter puss, Smokey, but .. nope .. no typical kitty cat piddle odour.
Today .. same couch .. luckily I have a blanket on it .. he just stood there and whizzed .. not a lot .. I'd call it a 'marking piddle'
We have another Westie, Lilly, almost 4 .. and Smokey the kitter puss (13+) all get along well, all are treated fairly and equitably.
I thought, obviously mistakenly, that once males were neutered, marking in the home would NOT be a happening thing. Am I wrong ?
Gotta get this 'nipped in the bud' as we are welcoming Westie #3 (Kate) at the end of Feb. beginning of March.

Any suggestions greatly appreciated ^_^

~M~