Relationship Boundaries

I have been very good friends with a guy going on 6 years. We used to live together in college and we’ve been friends ever since. He recently gave me an extremely thoughtful gift as a surprise. I want to write him a thank you note for the gift, and want to express how much I value our friendship/ him in general. I was thinking of thanking him and also writing, “there is no one like you. I hope you know how much you mean to me”. I just want him to know how much I appreciate him. I would never think twice about expressing something like that to him, However, he has a girlfriend who he’s been dating a few months and I want to be respectful of that boundary. Do you think that’s inappropriate to do/ say considering the history and context of our friendship?