It is possible he is getting pressure-induced urticaria
that is aggravated/triggered by the pressure of the elastic.
As he tried the 'no underwear' without help, it is likely he is getting urticaria (hives and bumps, red and itchy) at other times or places and scratching aggravates the problem and has got secondary bacterial infection.
cream is treatment for the local skin infection
but occasionally may require IV antibiotics if he is getting fever
Long acting antihistamines with calamine
lotion as local sooting effect is recommended for at least 6 weeks.
CBC, LFT, ANA, TSH fT3 fT4 are tests that are usually done in urticaria to see if there can be identifiable causes.