Back to basics

Oh how I wish I had a dog that pulled on the lead! Forwards that is, but not backwards! I have a problem with my 10-year-old male neutered Westie, which is really spoiling our walks. He persistently yanks my arm backwards. This is because he has noticed an interesting smell and wants to stop to investigate, but it's driving me mad.
At the same time I walk my eight-year-old female spayed Westie, who walks nicely on the lead and when I am walking with my husband and I take the female dog, the walk is altogether different because she is less interested in stopping. The male dog has recently pulled my husband's arm backwards with such force he really hurt his shoulder. This dog is very strong and sometimes it is unbelieveable that a small dog can pull with such force.
I have tried everything I can think of - standing with him until he has finished and wants to walk again, telling him 'no' firmly every time he wants to stop to sniff, yanking him forwards regardless of what he wants to do, trying to keep the lead slack and gently encouraging him to walk forwards again, and keeping his lead really short, so that he has no opportunity to pull my arm backwards.
Is the male Westie too old to retrain in the art of walking nicely and not pulling backwards? I would be prepared to take him back to training classes if you thought this would be helpful.
Jane Hull, by email