Men and women see things differently. This is how it is and how it always will be. While we spent time as young adults fantasizing about our perfect wedding, men were fantasizing about cars or the model on the cover of the Sports Illustrated swimsuit edition. It is what it is.
With that said, there are communication tools that can be used to get two people, of the opposite sex, on the same page. When it comes to discussing marriage there are several strategies to employ. Below are two of my favorites:
1.) Let him know that he is being heard by acknowledging what he is telling you. For example, "Sweetie from what you are saying I understand that you are stressed." From there set a date when he will be ready to move forward with the wedding plans by saying something such as, "It is not my intention to have you feel pressured or stressed by the idea of marriage. I know that it is a lot to take in all at once. Let's take a break from marriage plans, but let's set a date that we can start discussing it again." This is a diplomatic way to get him on the same page with you, but also gives him time to get over whatever he is going through.
2.) If you have tried the first idea or if he keeps saying that he feels pressured perhaps he is not at a time in his life where he is ready to commit. Let him know that if he does not want to set a date for your wedding that you are going to set a date for the two of you to break up. This may seem extreme, but it does, and had worked. If this is the option that is employed it must be done when you are very calm. Start by explaining that it is your desire to get married and since the two of you are unable to talk about it you have decided that it would be best to part ways in ___ months/years or whatever time frame works. There will be an initial shock from him, but he will realize that if he doesn't figure it out then you will be gone because you are not going to remain engaged for the rest of your life.
Best of luck!