My knowledge of rashes is related to eczema and dermatitis is another name for eczema. However if they are calling it dermatitis you need to find out what type it is treat it or avoid allergic contacts. For a doctor to tell you that you have dermatitis, is like a doctor telling you that you have cancer. You need to know the type of cancer you have to treat it: lung, breast, colon and so on. Is this a dermatologist that told him he had dermatitis? There are test sthat can be given to test for contact allergies, The Five Day Extended Patch Test, Google this test and read about the True Test Kit and what it tests for. I didn't develop contact allergies until middle age, one day you are fine and the next day your world changes. Has this test even been suggested? That said a symmetrical rash isn't related to any eczema I am aware of. Perhaps if you could describe it better or take a photo it would help. I really don't know what it looks like. By symmetrical do you mean the rash is balanced in shape, all being circles or ovals or that it is identical in pattern on each side of the body. Describe the sensation, most eczema's or dermatitis are itchy or painful.
He could have an allergic reaction to the nerve block I imagine. Have you checked the internet for side effects of a nerve block? Many of us learn of allergies to sulfa meds (I did), penicillin and the like because of treatment. Sometimes eggs can cause allergies in treatments. To begin with I would ask for a list of all ingredients in the nerve block he was given- both active and inactive. After that research each ingredient for allergies or reactions. It may not be common to react but for me I am always the 1% so I understand the difficulty in discovering a cause.
For more information about eczema/dermatitis go to dermnetnz, the New Zealand Dermatology website. It's better than any site in the states. Go to photos and start searching for any rash that resembles his. BE STRONG, these photos will break your heart and may terrify you, but you may find a similar rash. You will learn about the complication of some many forms of eczema leading to a confusion of diagnosis. These photos even for my 4 different types of eczema are much more severe looking than my symptoms were, but if you find any rash close, research that type and there will be assorted levels of rash.