Every fish has a different make-up, just like every plant does, so yes it's totally possible to be allergic to one kind of fish and not others. Most allergy tests only cover a small variety of fish and nuts and these are not done individually, so if your son only reacts to one kind which was not tested for, then it would not have shown up on the test. Also, be aware that if the tests are positive, many allergists find it easier to tell their patients that they are allergic to "fish" or "tree-nuts" and have them avoid the whole category rather than testing each individually to find out which ones they are allergic to. Also, some people react to certain food items when their immune system is being stressed (teething, illness, in combination with other allergens or following a separate reaction).
If the salmon you gave him was not 100% fresh, plain salmon, carefully check any and all ingredients in any marine/breading/oils etc used on it for his other allergens. Definitely consult your allergist regarding your son's reaction to the salmon and hold off giving it to him again. If you decide to try to re-test/re-introduce salmon in the future, consider setting up an in-office trial with your allergist.
I'm not sure on the seasonal allergy/rash combo, but it's probably possible. Double check all labels for changes in the ingredients of any processed food items you may be using. Also, look into any pesticide/herbicide spraying that has been done in your neighborhood recently as well as any changes that may have occurred in the ingredients in your cleaning/personal care products--especially laundry detergent and fabric softeners.