Young student doesn’t want to have all 5 fingers on the keys at once

We understand that each finger has a number and he will play with the correct finger but then take his whole hand away and then come back with the next one. We’ve been working on this for a few weeks and this has been a sticking point as I’m honestly not sure how to move through/on until he just…does it. Any ideas? He is a lovely 6 year old, his mom says he has some delays and is in OT and speech, but I have no idea if this is related.