I am not sure what you mean by 'not the other stuff' but red, raised itchy areas that disappear within a few hours and reappear at different sites is urticaria
Part of the urticaria in your son may be secondary to contact with tree/grass pollen as he has hayfever
, and you could check to see if this worsens at that time.
He can have long acting antihistamines like cetirizine or levocetirizine
for 3-6 months pre-and co-seasonally (during spring) and see what the response is.