I would probably feed for his current weight, but watch to see how fast he finishes the food. If he devours his food in a couple of minutes, that probably means he isn't getting enough. I would give him a cup more. Watch him, if he seems hungry, he probably is. The feeding guidelines on the bag is only a recommendation. You will know when Ghost has had enough, and he will probably leave the bowl when he is done. If he is that underweight, maybe feed him twice a day. Good Luck with him.
I did not know Ghost had diarreah. I would have him seen by a vet to check for worms or some other intestinal problems. They can do a quick fecal exam and let you know if he has any parasites. That could be why he has diarreah and is eating so much. What kind of food are you feeding him? That could be another cause.
I just read your initial post about his eye swelling up also. How is that doing?
Anyway, if Ghost has had diarreah for as long as you had him I would take him to the vet and have him checked out. Good Luck, you will get this worked out, and Ghost will be a happy healthy bouncy boy in no time!
I'm with Tootsie,,if he still has diareah he needs to be seen by a vet...If he has had a food change since you got him, or perhaps did you use a different food than where he was before? That could account for it, but he should be over that by now...