Mac was at the vet again this afternoon. He had an IV inserted, and some fluids injected.
He did look alot better after that. His problem seems to be a gastric problem, and the vet said he was dehydrated. This was what probably killed his other brother or sister.
This evening he ate a huge supper, and actually was playing with Scooter's tail while Scooter was sleeping. Mac is not totally healthy yet, but the vet feels that if we can get him to gain weight, and make sure that he is not dehydrated, he has a fair shot at becoming a fine adult cat.
Once again, I cannot say how much everyone's kind thoughts and words have meant to myself, my sister and of course Mac. I'll hopefully be posting picture of him soon, and I'll have pictures of Zoey on Monday.