State that you "highly recommend" the person or you "recommend without reservation" or something similar.
Letters from HR tend to simply supply bare bones facts, like the dates of your employment. What goes into a reference letter?
The font size should be between 10 and 12 points, so it's easy to read. Use business letter format.
Share your contact information. He and I met while volunteering as mentors for the local chapter of the Boys and Girls Club and he also offered his consulting services to my small business, helping me improve my website and increase web traffic.
To help them prepare, provide the job description and your current resume, and let them know as far in advance as possible.