There are two possibilities:
1. He is allergic (note this is delayed hypersensitivity
, not immediate) to the metal in the suture; like nickel allergy
2. The stress
of the surgery leads to the urticarial eruptions that manifest as the itchy red lumps which settle with antihistamines.
The first option can be excluded by knowing the type of suture and getting a patch test
done by an Allergist or Dermatologist
The second option can be treated with long acting non sedating antihistamines like Allegra
180mg once daily for a week; you can start this before the surgery if need be.
I hope that was useful.