This is pretty standard for many kinds of LOR. Many, if not most, college profs who write letters of rec for jobs or gradute programs want a draft to work from. Not only is it less work for them, it also gives them an idea of what you want to emphasize, and it reminds them of your qualifiactions and experiences.
You can use The Google to get templates and there are tons of them which, with minor adjustments would work quite well. I'd advise you to actually put a good amount of effort into it because it's possible much of your content will be used, and because if he senses you half assed it, he might do the same. You can't expect him to care more about it than you do.
