1）他能不能持B2簽證來美，然後和我在美國結婚？
-Yes.
2）結婚後，他是在這裏調整身份好，還是讓他先回中國等，我在這裏給他提出綠卡申請？
-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）他有兩個孩子，一個15歲，一個20歲，我可以幫他們一起辦綠卡嗎？如果可以，是和我男朋友一起申請嗎？
-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.
