How To Ask For A LOR?

Hi, I have been working as a medical assistant for the past two summers with a podiatrist who is very well know, having been on the Virginia Board of Medicine, and a prominent figure in diabetic wound care. Last year when I left to return for school he said "If you need a letter of recommendation let me know" but I told him I planned to return the next summer. Now that this summer is starting to finish up, I'd like to take him up on the offer. I know it would be a strong LOR because he would take the time to teach me things that he didn't have to. How should I ask him and if he agrees, how should I retrieve it. I'm only a junior undergrad so it wouldn't be used in an app for a couple of years. I have looked into Interfolio but not sure how it works or if it's the best option. He is also old fashioned so it would have to be simple.