Is he on flea prevention? Since he goes outside, even for a short time its possible that he has picked up fleas and these are bothering him and he is scratching when you are not looking. If he is not on flea prevention, I recommend using something topically. Some of the medications that I am familiar with are Revolution (good for preventing ear mites and heart worms too), Advantage (or Advantage multi) and Frontline. Since I work in emergency, there may be additional newer products out there that I am not familiar with so you can also talk to your regular veterinarian out the products they sell and recommend. Some of these are available over the counter and others you need a prescription for.
The reddish color to the water when you bathed him is likely blood from the flea bites. You can use a soothing oatmeal shampoo to make his skin feel better but I do not recommend bathing him no more than once a week as frequent bathing can make his skin dry and more itchy.
If he is already on flea prevention, it may be possible that it is no longer working as intended or needs to be used more often or in conjunction with another product. Are you able to post a picture of the bugs you are seeing?
Also, the environment needs to be treated as fleas can reproduce in the carpeting, bedding and upholstery in the house. As long as the house is not treated then treating your pet only will have a limited effect on the flea population. There are several products out there for the treatment of the environment but I recommend using one with an insect growth regulator in it. This will kill younger stages of fleas and not just adults. This will allow you to control the infestation in the environment faster.
Here is an article on fleas that you can read for more information http://www.petmd.com/dog/care/evr_dg_fleas_on_dogs_and_what_you_can_do_about_them
Does this answer your question about your cat? Please let me know if i can provide more information or you need clarification.