PS -- reactions to true allergies can lessen over time by staying away from the cause, but the possibility is always there is my understanding. Similar to those who test + via blood to Peanuts but never react ... they can react one day out of the blue since the antibodies are present and have formed due to the allergy.
C~
WELCOME to our Community. I'm not a doctor or medical professional so can only give you my thoughts on your excellent question. What are his symptoms to the NSAID's? As a sidenote, many who are allergic to NSAID's also have Aspirin allergies, too, so may be worth keeping that in mind.
Depending on his symtpoms, most likely an Allergist will determine whether they want to perform a challenge. And, if so, that challenge typically is done in the Allergist's office in case of an emergency.
Usually true allergies get worse over time and repeated exposure. So, usually, from what I understand if there's a true allergy it usually remains since the antibodies are always there. But I'm not 100% sure and only your doctor can really guide you and/or your boyfriend on the really accurate answers.
Did he get hives? Asthma? Rash? Guillain-Barre from the med? Joint Pain? Do they think it was a delayed reaction to the NSAID's or were the NSAID's given for the Guillain-Barre.
I think the level of his reaction will be a main factor on a challenge.
Just my two cents.
Hope this helps,
C~