Over years of having an established cat allergy but loving cats nonetheless I've found that I sometimes have a stronger reaction to cats I visit infrequently than to familiar cats. There's a good chance that you'll get used to him over time, but in the meanwhile you should be careful to wash your hands after handling him and also could consider giving him a bath to reduce his dander levels -- as he settles in to his new diet, surroundings, and new family this will probably improve anyway, it seems to me that dander is worse with a cat who's had dietary and psychological stressors in the recent past.
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Good luck, anyway.