Any experience using Allerpet/C for cat allergies?

My boyfriend and I recently adopted a little kitty. Within 30 minutes or so he was showing signs of allergies. (Stuffy nose, sneezing.) By the 4th day, it had gotten so bad that his chest hurt and his throat was constricting. He was having a hard time breathing.
We sent the kitty back to my Mom’s while we are trying to figure out what to do. We are both heartbroken thinking that we might have to find her a new home.

Does anyone have any experience using Allerpet/C? It seems like it would be the perfect solution, but we don’t know if it really works.

We already tried switching to crystal cat litter. (Which has almost no scent or dust.) We used hypoallergenic/shed control wipes. Vacuumed. Etc. The only thing he didn’t try was taking allergy pills. He took one after we already dropped her off at my Mom’s, and it seemed to help, but we don’t know for sure, since he isn’t around the cat anymore.

He has never shown signs of being allergic to cats before. (Even after spending the night in a friends one bedroom apartment with her three cats.) Could it be because this cat is licking herself more than usual? (She is a bit of an OCD/stress groomer.)

Any advice for someone living with a cat and allergies?
If he took an allergy pill every day, for as long as we have the cat, would that help?

Any advice is welcome. We are desperate.