Dear Mr. and Mrs. {Surname},
Thank you for raising an amazing young man. {Boyfriend} impressed me with his incredible work ethic and sense of responsibility when I first met himâI knew that he had to have come from a wonderful family.
Speaking of, thank you for making me feel like a part of your family. Being included in family activities and gatherings means a lot to me. Thank you for taking care of me and making sure I eat well every time I come over.
Thanks for everything! I'm very happy to have met you both.
Yours respectfully,
{Your Name}