The first issue is that you haven't talked about is IF he wants to be part of your business. What are his long-range plans for his education and career?
Second -- he's 20 years old. Is he old and wise enough to know what he really wants?
Third -- What does your brother have to bring to the table? Can he pull his own weight?
I'd start with you owing the business. In time, set up a program where he can buy in either with work or money. Give him a chance to prove himself. It must be fair to you. If you give ANYTHING to another person that they don't feel like they deserve, they WILL end up hating you for giving to them. Don't set your brother up to either be angry with you or overly dependent on you. You want him to organically grow as a person. You can be a big part of that as his big brother. He'll need space to explore the world. You must give this time to see how it develops.
Second -- he's 20 years old. Is he old and wise enough to know what he really wants?
Third -- What does your brother have to bring to the table? Can he pull his own weight?
I'd start with you owing the business. In time, set up a program where he can buy in either with work or money. Give him a chance to prove himself. It must be fair to you. If you give ANYTHING to another person that they don't feel like they deserve, they WILL end up hating you for giving to them. Don't set your brother up to either be angry with you or overly dependent on you. You want him to organically grow as a person. You can be a big part of that as his big brother. He'll need space to explore the world. You must give this time to see how it develops.