If you are living on your own and have been then your mother really has nothing to say about who you date or live with, she is just being a concerned parent but it's only what you want that will matter in the end. Tell you mom that though you appreciate her concern that your are of age and it's your life to live and that you have to learn from your mistakes on your own if you do make any mistakes. as far as your boyfriend not working if he has a legitimate illness or physical condition then your mother need to understand his limitation or maybe you could do some research on his illness and also give her some of your own experiences with his illness and maybe she can better understand what he has to go through and why it's going to be hard for him to find employment maybe you should tell her this.
Your mother is going to be your mother no matter what and she will always worry about you no matter who you are with so just take that into consideration but also maybe talk to your boyfriend about applying for SSI that way he can contribute to the household and maybe your mother will feel better about him. It's hard when you feel like you are in the middle of your mother and boyfriend but you have to make sure that your life with your boyfriend stays separate from your life with your mother try not to get her involved in your relationship with your boyfriend this could alleviate some of the issues she has with him and if she still doesn't want to give him a chance then maybe your just going to have to let her have her opinions and take them with a grain of salt. Just talk to your mother one on one maybe ask her out to dinner or go to her house alone and talk with her and let her know in a polite way that you are no longer a child and she has to allow you to live your life how you see fit. It would only be her business if he were beating you and treating you badly but from what you have given me her main concern is his not having a job. The job market is hard enough with the economy let alone for someone who has a medical condition.
Your mom doesn't understand because it's easier to judge something you know nothing about that is why I said maybe you should do some research about epilepsy and give it your mom to read. Parents don't think anyone is good for their child but them. Your boyfriend should still consider applying for SSI until he can get his business up and running and will be able to contribute that way, either way you may have to talk to your mother and explain to her that this is your life and tell her that you are happy with your boyfriend and you want her to be happy for you.
If he is contributing then I don't understand why your mother thinks he is a deadbeat. He is doing what he can do and he does have a desire to work and own his own business, she should see the positive and not the negative. I think you should just allow your mother her opinion and you both agree to disagree, this will make your relationship less strained and until she really gets to know him and his character then you should keep your relationship separate from you and your mothers relationship. Parents are always going to have their opinions of who you are dating or living with, no one will ever be good enough for you, even if you dated someone that had a job and had his own place I'm sure your mother would not approve of them for some reason or another also.