It can be really hard to deal with unsupportive people. I realized after my 21-day
Water Fast last year that it is best to keep things as much to yourself as you can (except for posting on supportive sites like this). If it's a question of a rare social situation, feigning a small illness always seems to be the best way to avoid eating or drinking anything but water. Otherwise, avoiding those situations altogether is the easiest way. In my own experience trying to explain fasting to a group of people who are planning on eating food in front of you can make them really uncomfortable.
On the other hand, dealing with an unsopportive boyfriend who is around you all the time can be tough. When I did my 21-day
Water Fast last year, my boyfriend was supportive but cautious. When I finally reached the end, he was amazed at how long it really was that I had gone without food.
I am preparing for a long fast to do in the next couple of weeks and he is 100% behind me and a little excited for me. Last time I know he was a little worried, but I didn't die and nothing bad happened to me so I guess the proof is in the pudding.
The best you can do if talking to him about it doesn't work and showing him proof that other people have done it and survived marvelously, is just to decide for yourself that you are in charge of what you eat or don't eat, which is true whether you are fasting or not. Once he sees that you are alive and well after getting through some of it, perhaps he will be more supportive.
Perhaps also he feels uncomfortable with eating alone or feels some sense of loss that you are not sharing meals with him or maybe he feels left out of your fasting experience. I think it could be lots of things. He may just be scared that he might lose you if he is one of those people who is convinced that you will DIE if you fast (there are lots of people like that out there.)
So perhaps you can help him understand things and perhaps you won't be able to. In the end, if you want it badly enough you will do it no matter what people say or do around you. The benefits are too numberous and the experience too special to let the world stop you.
Fasting has definitely taught me that you cannot always go on what are the accepted wisdoms about things. You have to find certain things out for yourself.
And you can always come to this site for support if you need it.
Best of luck!