Then unfortunately, he will have to start all over again. He can get a B-2 tourist visa to visit, but not to stay. There are 5 ways to come to the U.S. to live permanently. They are through family, through employment, through asylum, through investment, or through the lottery.
For family, it must be immediate family such as U.S. Citizen spouse or U.S. Citizen children over 21 in order to be able to come immediately. U.S. Citizen or Lawful Permanent Resident parents or U.S. Citizen siblings, or Lawful Permanent Resident spouses can also help, but the wait for that is about 4 to 12 years or so.
Through employment, he would have to generally prove that he would not be taking away a job from a U.S. worker and the less experience and education he has, the harder that is to do.
Through asylum, he must prove that he would be persecuted, tortured or killed if he stayed in his country and that this will happen to him because of his nationality, race, religion, political opinion, or membership in a particular social group, not because he would run the same risk as anyone else in his country to fall victim to a crime or bad economic conditions.
Through investment, he is looking at generally $1,000,000.00 for a residency, or $500,000.00 in an area designated as a high-employment area, plus in both cases the investment has to create 10 or more jobs for U.S. workers.
And through the lottery, it would be very difficult to win one of the 50,000 visas available per year and some countries do not qualify.
Then there are some visas available where he can stay temporarily and then later change to Lawful Permanent Residency if he qualifies.
Some of the visas available through employment (if he has a job offer and he qualifies) are:
H-1B for professional workers
H-2A for seasonal agricultural workers
H-2B for seasonal nonagricultural workers
H-3 for industrial trainees
Then there are the F-1 and M-1 for students and J-1 for exchange visitors.
And then there is the E-1 for treaty traders and E-2 for treaty investors (these types of visas are not available in all countries).
Here is a link to most of the available non-immigrant visas:
I am truly sorry. He does not get any credit for having had Residency before and now that he is over 21, it will take awhile for a family member that is not a U.S. Citizen spouse to petition for him. Please let me know if you have additional questions and please do not forget to rate my service to you (not the state of the law) as that is the only way that I can get credit for my assistance. Even after you rate the service, I can still answer additional questions for you without additional charge. If you do rate me positively, a bonus is always appreciated. If you would like to request me in the future, just go to http://www.justanswer.com/law/expert-guillermosenmartin/. Thank you!