Franky, thanks for your question!
Not all flea/tick preventatives are created equally. I'm going to provide you with a link to visit regarding what "off brand" flea/tick preventatives can do to our beloved babies. I would recommend getting your companion on a top-quality preventative like Frontline. This is the gold standard in veterinary medicine. Once you've picked a great product like Frontline, you won't need to use a flea/tick shampoo. You can use a regular, good-smelling shampoo (of course, pet safe) instead.
Cocker Spaniels are known for their ear issues. This may be part of the smell you're detecting. An examination by a veterinarian would let you know if he's got ear issues that need to be taken care of apart from the itchy skin. The ears should be a nice, soft pink with no discharge and no smell. Any inflammation, irritation, redness, discharge, overgrowth of hair, etc indicates that your pet has something wrong with his ears and he needs to see a veterinarian. Here's a picture of a healthy ear: http://inlinethumb21.webshots.com/21204/2595053150102620448S425x425Q85.jpg
You can also check for dry skin by rubbing your hands against the grain in his coat. If he has dry skin, you should see white flaking. This can be resolved by feeding a higher quality diet such as Innova, Merrick, Canidae, etc. You can also add fish oil to your pet's diet to help condition the skin from the inside out (there are also studies that show that fish oil is good for the liver and heart).
Good luck with your companion!
Sami
Pyrethrin/Permethrin Toxicity: http://www.petplace.com/dogs/permethrin-and-pyrethrin-toxicity-in-dogs/page1.aspx