Love letters are generally reserved for when you are already in a relationship. If you are using it as a way to start a relationship then I would be careful about the wording because you could easily word something wrong or he could take something the wrong way and being a letter there would be no chance to explain.

If you can't summon the courage to kiss him then just suggest another dinner together but ask him if instead of as friends it could be a date. From what you say he seems to be attracted to you but maybe he is in the same situation that you are in that he is afraid to make a move and ruin the friendship. Its better to know then to miss an opportunity.