Guarantee issues when going abroad

If he is over 18 years old, you cannot sponsor him (no guardian is needed), because he is not an immediate relative, and if he is inclined to immigrate, it is better to apply for it yourself.

If he goes out, his uncle can hire him.

It is very difficult to get a work visa, unless you are officially assigned by the government.

To be able to apply for a green card just because your work visa expires, you must have a US company hire you, or you must have been out of work for more than 10 years.

He has nothing to do with his uncle's children and wife. If you insist on having a relationship, it will be difficult for you.