I would give it more time. Most likely, the baby doesn't have an opportunity to PRACTICE these skills at home. He's probably carried everywhere and his parents are probably still bottle feeding him because they want to, not because they have to. If you feel like you don't have the time, patience, expertise - whatever, to be able to put a little more engery into this little boy then you could let him go. Because that is what you'll have to do. Within some time, he will learn to follow the others, he will learn to crawl, stand, eat and so on. My son stood the fist week my older charge was here just because he wanted to follow along. Your little man just needs to opportunity to be TAUGHT these things. To be given the OPPORTUNITY to learn these things. HOWEVER, it will take a little more energy and time on your part. It's not a bad thing if you decide you don't have that to give at this point. Being a new daycare is HARD, lol, I hear you on that! If you don't think this little boy is a good fit, or you don't want to spend the extra time, then let him go. There are plenty of places he can go, so don't feel guilty. However, if you decide to keep him, it won't be long before he is moving along with the others. You just need to be patient and consistant. Perhaps even mention to the parents that he should AT LEAST be crawling by now and that he should see a Dr. just as a precaution.
I feel like I'm sounding like I'm telling you you're a bad person if you don't keep him lol but you're not. It's all about YOU and YOUR daycare. You're running a BUSINESS. If you don't feel like you want/can do it, then don't. If I've learned anything from my little man, that is that you need to be careful in who you choose for your daycare. After all, we're only human and can only do what we can do.