Does the area stay as red, a couple of hours after he's gone to roost? Was the crown of his head always somewhat prominent, or is it swollen as well. Did this progress over several days or happen suddenly? Has he been sparring with other toms? Benadryl is something to try (I'd go with same mg/kg for use in allergic reactions with humans). Could be anything from a bug bite/circulation problem/infection. Check him at night, just to see if there isn't some decrease in swelling when he's quiescent.