Start by asking him (let him show through video) about the type of visa he got. It most probably is a type D visa, which enables him to stay in the US for 29 days and is valid up to a couple of years. Tell him you just want to see how long he can stay in the US with his visa and for that you need to know the type.

The main anchor point for this visa is, as others have pointed out, how he showed his intent to return to his home country. Usually, there are a few situations that guarantee it for the embassy: having a commercial business, having wife and/or children or having proerties of his own. However, one will never know what is the case, because it is, in the end, depending on what the embassy personal decides.
So, all of what he says can be true, and probably is, however, it won't bring you an inch further. It is quite common that tunisians are employed by travel organizations, eg. in Turkey or other countries or on cruise ships.
Having a technical degree from a tunisian university says nothing. Firstly, there are only some universities with internationally recognized degrees, secondly the tertiary education in Tunisia suffers greatly from the lack of quality control, However, a technical study without a firm grip on the english language is what I'd call rare ... but possible (tunisian universities usually educuate/train for a job in the public sector in Tunisia). Here, again, closer information would help you to decide on the value of this degree. In addition, you would need to know where he spend his time between the time of the degree and the actual working position.
The job market in Tunisia is problematic unless you have good connections and/or a high degree and/or studied abroad a couple of semesters - but for someone with a academic degree, it would be unusual to work as a helper on a cruise ship at the age of 32, as this job will bring him no closer to a "better position" at all - unless the better position will be a result of meeting a citizen of a western country (either through job offer or visa possibilities).
In the end, there is probably no other way than to meet him in person in Tunisia. You paying your flight and hotel (where you return at night), and he paying the other expenses would be a fair deal ... for your advantage, you will see parts of Tunisia.

Be aware, though, that this is a holiday visit and you will see everything with holiday eyes and be shown things that he selects.
Nevertheless, after such a visit, you will have more information that might make your decision easier.