I think you have a good idea. I think a detailed description of your work with him would be a good start. In particular, try to highlight things about yourself (honestly, of course) that adcomms would be looking out for -usually things such as 'takes initiative', your leadership abilities, you ability to do independent research, your dependability, your intelligence and whether you would be successful at grad school. Your LoR writer would probably edit the letter if he thinks some things need to be changed, but I'm thinking those are the kinds of things that a good LoR would possess.