If they are that age, I would do as CC says and put the little one in a box and put a small amount of kitten food in there to see if he/she eats it. They may be of an age that they can take solids.
Definitely place the box somewhere warm, dry and safe and lie in wait to see if a mum appears or the other kitten returns. If no sign of mum, then it will be necessary for the little one to taken in and if no solids are eaten then a kitten milk (cimicat, Lactol Gold for Kittens, or similar) will be needed to feed them. They will also probably have fleas if they have been raised outdoors and fleas can be killers on little ones, so they will need to be treated but be careful as many flea treatments are dangerous to young kittens. I would think a vet check will be necessary to give them the once over.
Whereabouts are you? I handrear orphaned and abandoned kittens so if you need any help, pm me.
