New Vegas Worries - Advice?

Vegas is now somewhat constipated. A little while ago he was walking around with a piece of poop half in and half out! What is in the litter box is somewhat dry and pebbly. It's odd, because he's actually getting more water now than he was before we went to the vet. I'm soaking his dry food in water mixed with a powder for the arthritis.

I stopped the metacalm a week ago because the lump on his leg was down to almost nothing, so it can't be that.

Vegas is very picky about food now, and refuses pretty much anything except what is familiar. Any advice on something I can do for him?