What is your question?
I would probably mention it to him one more time and say something to the affect "I was really looking forward to reading what you were going to write me, are you really going to disappoint me?" and see what he says. Encourage him to send it to you as an email since that's probably easier than writing it on paper and see what happens. If he doesn't do it, it might not necessarily be a reflection of how he feels about you, but have a lot more to do with the fact that he may have thought he knew what he wanted to say and now cannot figure it out, or that he's not sure you will like it, or he doesn't think he can write it as good as he thought he could. Either way, the more time passes, the less importance he will place on it because Valentines day is over. Guys just don't think like we do, and they don't place the same importance on things that we do. The good news is he made you a nice dinner and bought you a beautiful, meaningful gift that you can cherish for a long time. If nothing else, thank him for that, and if you have to let the letter go until net time, do that.
I hope this helps. I welcome your thoughts, let me know if you want to talk more.
You should mention to him what I told you to say. You can't assume that a letter would take him 20 minutes to write, you also can't assume that this is something that's easy for him. Men are just not built like we are. My husband told me the first year we met he was going to write me a love letter, it's been 8 years, still no letter, but plenty of other good things. I know you want the letter, and I know what it probably means to you, but you can't hold onto it if he just doesn't feel like he can do it. It's what he does not what he says that really matters anyway and if he's a good man otherwise then yes, you may be putting too much emphasis on it. I don't think how he feels about you is tied into whether or not he writes the letter, but if you keep pushing it and he keeps resisting, it could become a point of contention in the relationship and you don't want that.
Of course, that's why I said to ask him if he is going to disappoint you (or break his word to you). It will make him see that it is a disappointment to you, and also make him feel like he's let you down a bit. I'm just saying don't make this into a deal breaker.
I hope this helps, if so please remember to click the green accept button so I can get credit for my work with you. As always, feel free to ask for me by name anytime you have a question, I am at your service.