Your son may have a rash for a few reasons. The first is an allergy. Neoprene contains chemical allergens that may cause contact dermatitis for some individuals. Most divers with this type of allergy usually wear a skin (lycra) as a physical barrier under the wet suit. Another reason may be chafing. Does the wet suit fit him properly? Is the rash in areas where the wet suit rubs against him, such as the neck? If so, you may want to invest in a suit that fits him well. I didn't look to see where youre from, but it could also be a bacterial or yeast infection. When you say a "rash What kind of rash is it? Are there pustules? Is there any oozing? Are you diving in an area where the water may be contaminated? I know the South Coast beaches of Oregon have had a problem with contamination and some are even closed. Even if it's not an infection, make sure he's rinsing his equipment adequately. Often times when people have been diving in fresh water they don't bother to rinse their wet suits...make sure he rinses the suit ever time he uses it regardless of the water type. You may also want to regularly soak the wetsuit in baking soda and or a commercial wetsuit soap to kill the bacteria. Make sure he's hanging it up to dry.
Hope this is helpful to you...it's great you and your son are diving together, it's a wonderful family activity!