Baby bunny needs lots of TLC

Two days ago while excavating in the yard, we found this teeny-tiny newborn bunny.  His mama must have had her den in the ground right where we were digging, and this little guy survived but lost his home and family.  I really didn’t expect him to live for more than a day without his mother, but the kids insisted we give him a chance by feeding him with an eyedropper.  Though his eyes are still closed, he is beginning to squirm and wriggle around; we’ll just have to wait and see if our help is enough.  Does anyone have any advice for us, such as how often we should be feeding him, and what is the best milk for him?