Plan of attack

I have three lovely Westies - two elderly ladies, one 15 plus, one 13 plus, both with medical problems since before Christmas, limiting their ability to go out for good runs, and a boy of three, a really affectionate and friendly dog whose only dislike is fluorescant jackets.
My little boy and I go for long walks in the park, common etc and he meets and greets everyone amicably. He loves playing with all the other dogs, whatever the breed, big, small, with no problems.
However, mid-December I met a lady with a delightful black male Lab, a lovely dog, but my boy took against him. I was amazed. He flew at this dog, barking and attempting to bite its bottom and tail. The other dog fortunately went on ahead with its owner - rushing away from us - with me chasing to catch my dog and apologising profusely.
I have kept an eye out ever since, but was caught out last week when the same dog was in the care of a professional dog walker. It all happened again. Luckily my dog failed in his attempt to bite the Lab's tail and bottom, but the person in charge was understandably livid and said I had a dangerous dog, out of control and he should be muzzled. My protestations that this was an isolated incident didn't placate her and I fortunately caught my dog and went in the opposite direction. My Westie then went on happily to play with all the other dogs he met, including Labradors with no problems.
We nearly met again, but I saw the person coming and caught my dog - but believe it or not even from a distance he knew and once again was tugging at his lead barking. I am at a complete loss to understand this. It sounds stupid but could this dog be smelling of something that my dog dislikes. I am completely stumped, worried and being very careful and aware when I am out. I feel so awful as the other dog is so nice.
Any theories or answers would be much appreciated.
Jillian Paul, Reigate, Surrey

Karen Wild, Canine Behaviour Counsellor, says...
It can be such a shock to have your dog suddenly decide that another dog is not going to be one of their usual playmates! Please don't feel bad - I often come across cases where a dog has taken a particular dislike to a specific dog. It sounds like your Westie is attempting to chase away what he interprets as a threat. It may be that something about the other dog (or a similar dog) has 'spooked' your Westie in the past. This could indeed be scent, although there are many other combined influences on what your dog is reacting to. It may be that this other dog has or has not been castrated, that it may 'stare' at your own dog or offer other unwelcome body language - sometimes, even just size or colour of another dog can affect response. This said, it is essential that you look at regaining control of your Westie so that the other owner does not feel that your dog is going to hurt theirs. Calmly working on a rock solid recall command, with your own dog on a long line so you can reinforce the behaviour, is a good idea. You should also work on desensitising your dog to the sight of the other dog so that he is rewarded for calm behaviour and begins to feel that the appearance of the other dog is actually enjoyable. I would consult a professional behaviourist who uses kind methods, as you do not want your dog to feel this way about other dogs in future. If you take proper precautions now, you are less likely to be accused of your dog being out of control in public. If you are concerned that your dog may bite, you should consider a muzzle, at least in the meantime while you are calming your dog and re-training reliability to your command.