Rejected an offer and then became interested (a bad decision I made)
Yes, it is a good idea to ask. But explain that you now think you made a mistake in rejecting the offer. You might also want to mention your longer term goals, which I assume include further education. He can help you with that if he has a good sense of your needs. If your work for him is good enough, he might also have some incentive to help you along. One hopes for such an outcome, anyway. 

But yes, ask. Silence gains you nothing, nor him, either.