It's probably just age creeping up on him. What type of glucosamine product are you using on him? How long has he been on it? Those products don't work right away. Oftentimes you don't see any results at all for at least 30 days, and you don't get to peak performance until the pet has been taking it for six months or more. So unless he has been on the glucosamine for half a year or more, I wouldn't give up yet and say that it hasn't done much. I used to use a product called Cosequin for my Australian Shepherd when she started to develop arthritis in her hips, and while it took a good few months to see any type of difference, it did finally work, and it worked fairly well and bought her another two years before her hips got so bad we could no longer let her continue with the pain and difficulty getting around.
Next time you have to take him to the vet for anything, mention it and see what the vet says. Or, if you have a good relationship with your vet, call them and just ask them on the phone what they think it could be.
Ghilly
It's very difficult to say what this is. Has it always been the case, or has it developed recently? Has there been any injury, even a past injury, to the legs or spine, or is he on any medication (apart from the glucosamine)? Hopefully there will yet be more advice from others who may have a better idea.
I have seen dogs (usually Jack Russells) who 'trembled'. Healthy, energetic and happy dogs, those were. Nothing wrong with them. Obviously I'm not assuming this is the same, or what may be the cause with your dog, but I thought I would tell you that, just in case it IS possible this is nothing to worry about....(?)