Let him CRY it out. I would go in ONCE probably around the 30 min mark and with no eye contact, shhhhh him, lay him back down and leave him there the remainder of naptime. I would also have him in the furthest away room in the house, with a loud fan or white noise machine on as well. With transitioning babies who cry non stop for the first little bit, its important that we have our breaks from them. He may take a couple of weeks but will get used to it.
Something else to think about... have a time frame in your head that you can handle listening to the crying. Mine is usually at the 2 week mark I tell parents that this is their 2 week warning if the crying doesn't stop their little one isn't the right fit for my daycare.
Good luck to you!!!