1)他能不能持B2签证来美,然后和我在美国结婚?
-Yes.
2)结婚后,他是在这里调整身份好,还是让他先回中国等,我在这里给他提出绿卡申请?
-Technically he can file I-485 in US to get the green card after marrying you, but it is very risky because USCIS may think he has the immigration intent before he enters US using B2 visa and thus, he might be considered to have fraud during B2 visa application which will make him big trouble. So, you had better file I-130 for him to apply for the immigration visa in China using consular processing after you marry him.
3)他有两个孩子,一个15岁,一个20岁,我可以帮他们一起办绿卡吗?如果可以,是和我男朋友一起申请吗?
-Only the child with 15 years old can apply for immigration visa (green card) under your sponsor if you marry his/her father before he/she turns 16 years. You file I-130 for this child as you do for the child's father after you get married. The child with 20 years old can not be qualified for green card application under your sponsor. The father (after getting green card) can sponsor him/her green card later using F2B category which may take about 8 to 10 years for this child to get the green card.