If you want to keep him, I would be willing to bet you would have to target feed him at least daily, just to make sure he gets the nutrition he is supposed to be getting. Sand sifters starve to death slowly in home aquariums. Even bigger tanks (say up to 90 or 125 gallons). They can often live for about a year in a tank, and be starving for the last 6 months of their life. They will start to turn darker in color and then, as stated before, will lose a leg one at a time to attempt to save the core.
They are so readily available in LFS's, and so cheap as well. I can only assume that is because they are so easy to catch in the wild, becuase, IMO, they are really not good for home aquariums, as home aquariums and sand beds will eventually become depleted of the organisms they feed on because they will not be able to reproduce fast enough to feed the sand sifting star (stars can cover a lot of ground in your tank).
I'm sure you knew none of this when you bought him. Most people just think that stars look cool in their tank, and they do. I didn't know much of this at all when I first started looking into them. I almost bought one, but then I researched it and this was the info I found out about them. Stores try to sell them as algae eaters as well, so people end up buying them thinking they will serve that purpose.
Good luck with what ever you decide to do. But ya, if you keep him, plan to target feed him very regularly. They are constantly eating in the wild. Sorry we had to ruin a purchase I'm sure you wer happy and excited to make.