I can't be sure, of course, but your husband may have an allergy to the long waves of ultraviolet rays from the sun (the ones that cause tanning but not burning.) This is called PMLE (polymorphous light eruption.) It is pretty common. Prevention is hard, and involves using a sunscreen that protects against UVA (there are many on the market.) I suggest a consultation with a dermatologist who will know what PMLE is and what blood tests to run to diagnose it. You might want to take some pictures of the rash at its worst, in case it has subsided by the time you get to see the doctor. That's my best guess from a distance, but he'll need to see a doctor for definite diagnosis.
Best.
Dr. Rockoff