From my experience, there shouldn't be any reason to get rid of the cat. I myself have asthma which is set off by anything I'm allergic to, which happens to be cats, dogs, trees, flowers etc etc. I have 3 cats, 1 guinea pig and 1 sugar glider. I'm allergic to the cats, I'm allergic to the guinea pigs bedding, luckily I'm not allergic to my sugar glider. I'm able to deal with it by simply taking my meds and I haven't had a problem. Obviously, your grandson may have more severe asthma than I do, but I still see no reason to get rid of your cat. You're lucky enough to able to keep your cat in a closed off area away from your grandson. With good hepa filters and regular cleaning, there shouldn't be any dander getting near him except for what you bring up on your clothes, which I would doubt would do much if he's taking his meds. I would give it a while, see how things go on his meds before doing anything drastic.