Unknown Body Rash

My husband broke out in a full body rash on Saturday.  It covers head to toe.  The rash looks like slashes as if he ran thru 6 feet tall bushes.  The slashes are very dark red and do not change color when touched. There are thousands of them on his body-too numerous to count.  Some are tiney and some are as large as 6 inches by 1/2 inch.   Now (Tuesday) he has extremely small blisters on the slashes on his hands and feet.  His hands are swollen and he ran a fever of about 101  Monday.  We went to the urgent care and they had no idea what is it.  They gave him steriods and clarition.  He feels tired and aches.  My concern is not only for him but for our son who suffers from a compromised immune system due to a degeneration disease( Riley Day Syndrome)  Any ideas what this might be?  We would be thankful for any information.  Dr Wife