I need some advice please
Judging from your account of things it doesn't sound like this man is a devout JW, so it is possible that over time you could get him to celebrate holidays (though I personally wouldn't bet on that happening.) I have trouble believing that he will legitimately refrain from pressuring you to become a JW down the line, it may be subtle but he almost certainly will.
If he does not though I would advise you expect it from his family. They are trained to proselytize and love nothing more than an audience that can't simply ignore them. If he isn't a very active member himself his family may even try to convert you to get to him. This process will begin with something called "love-bombing," during which JWs are going to shower you with attention while they are testing the waters by overtly or covertly trying to discuss religion with you. If they think you can be converted they will continue the love-bombing until they succeed. If they determine you aren't "receptive" they will likely stop and have very little to do with you. However, expect the same "testing of the waters" with any JW you come into contact with.
If I were you I would consider carefully whether you can handle the above situation and the situations outlined by other people in this thread before you commit to marriage with this man.
You said that this man is your first boyfriend and that you've been together for five years, so because you have only ever been with one person, I would also recommend some serious introspection on your part. Determine for yourself what you really want in a life-partner, and don't purposely list the qualities of the person you're already dating. (Ideally, this is done while one is single.) Try to be balanced in this exercise, though, don't imagine yourself with a flawless person because that will never happen. Be sure to include the objective flaws you believe you can live with long-term. If you want to try this but are having trouble starting you could take a look at the Meyer-Brigg personality types and see which one in their strengths and weaknesses most closely fits the type of person you would consider as a life partner. Some people are lucky and find the right person the first time, some people, like myself, don't realize how incompatible they were with their first boyfriend/girlfriend until after they have broken up.
In the end, though, I would say the best way to tell whether a person is right for you, is whether or not your happiness is a priority for them. Two people can be good people, even great people, but if they don't both give priority to the other person's happiness, then one of them will almost certainly be miserable.
JaMu, he sounds like a nice, accepting guy - I can see why you are hoping to find a way for your relationship to work.
He has said he just wants you to let him practice his religion. That sounds like a reasonable request. Does that apply to both of you? Will he let you practice your religion and/or other pursuits? Would his view of you change (for the worse) if he knew you were in contact with ex-JWs or those who question his religion? How much of your life, your views, your hopes will you have to keep from him?
What if he decided to become more fully involved in JW organization? How could that impact on you? You would be viewed by his fellow believers as "an unbelieving mate"?