I believe you Chrystel. You're absolutely right. An allergy can present itself at any time, it often depends on exposure and he may have been allergic his whole life but never was exposed much to rodents so he may not have known. I have developed allergies to certain things recently that I was never allergic to before, I even have a small reaction to our guinea pigs if their nails scratch me. Also, we have a good friend that comes by once or twice a week that cannot be anywhere near the guinea pigs, his eyes swell up and he sneezes like mad.
Best of luck in finding a good home for Gamoutch, I can see you're very worried about it!