You can look at the link below for all the information that you need on the visitor visa:
Essentially, he must have a valid reason for visiting, must have a way to support himself without working illegally, must have significant ties to his home country, and must have the intent to return home.
A letter of invitation helps with some of this as it will help to show why he is coming to the U.S., how much time he will need and that perhaps he will have a place to stay so that he doesn't have to spend money. Usually you want to show that he is going just for a few weeks or so. If it is longer than that, immigration will wonder why and will become suspicious. They are almost sure to deny someone that says they want to visit for 5 or 6 months unless they have very strong evidences of everything else I listed above. But keep in mind that a letter of invitation is a small part of the process and isn't even a requirement (people mistakenly believe it is the most important) and it can sometimes be used against the applicant. For example, if the consular officer believes that the applicant will have too much support meaning that they could stay illegally in the U.S., the officer would deny the visa. Sometimes it is better to not even have a letter of invitation. The most important part of a case is to have strong ties to the home country.
So your letter should state why you are inviting him and that you will provide room and board while he is here visiting. Not much else is needed from you.
Now as to some of the ties, he will need to show that he has significant close family at home, property not easily sold, and a good job for a good company that he has had for awhile that he would not normally abandon.
So if he is young, and without much money or any property or a good job, he would probably be denied. They would see him as a risk for staying illegal. Also, persons that are planning to travel with their spouse and children are a risk as well because some of their significant ties are actually going with them.
I mean some people have been approved with less, but I am trying to give you the idea for the best chance.
Please let me know if you have any additional questions. I would be happy to answer them for you without additional charge, if not, please do not forget to click accept. Thank you.