I don't want to worry you but I echo the PP's who mentioned Parkinson's or cancer. Scientists have found that the lady who smelled Parkinson's on her husband was truly smelling it. As in blind tests I think she got it right 100% of the time!
Does your husband know that you are smelling something not nice from him? I know I would find it hard to bring the subject up with my husband if he didn't mention it first, but as a PP suggested, maybe your husband washes both himself and his clothes so much because he has smelled it himself? Having said that, I do think it can be quite difficult to smell ourselves, I think our noses quickly get used to our own smells!
I honestly think that it would be a good idea to get your husband checked by a Dr, but of course you can't tell if someone has Parkinson's just through blood tests. Has your husband had any other noticeable changes such as:
being more clumsy or having slower movements when he is walking? If you get a chance without being obvious, can you check if both his arms swing by his side when he is walking (whatever pace of walk he has), and do any of his other movements seem stiffer or slower? Have you noticed if his hands shake whilst at rest, say if he is sitting in a chair watching the TV or listening to music and his hands are relaxed, do they show any signs of shaking or twitching? If one or both hands shake only when they pick up something like a full cup of tea, then that probably isn't an early sign of Parkinson's (most people probably think of shaking hands etc as being the main sign of Parkinson's, but actually you can have Parkinson's without having a tremble at all - although one would have probably developed by the time the disease progressed to it's later stages).
I am only asking these questions because if you can observe him for a few days before he sees his GP, and you do notice any changes like the ones I have mentioned above, then the GP would be more likely to take it seriously (a good GP should anyway, but unfortunately I do not have the same blanket faith in GP's that I used to have), and refer your husband to a Neurologist - a GP can't diagnose Parkinson's. In fact there is no failsafe way to diagnose it yet.
Of course, except in patients with early onset Parkinson's, another difficulty in diagnosing someone with it, is that a lot of these symptoms happen to us as we age anyway, but some of them are quite distinctive, and need a Neurologist's eye. In fact I do wonder if it is just old age that you are smelling on him, so many things change and slow down as we age, that it wouldn't be a surprise if our smell changes too. Maybe with a man the lessening of testosterone could change the smell of his pheromones? I find the human body absolutely fascinating, but that is by the by here!