Your prenuptial agreement is irrelevant to immigration. If you want him to join you in US, YOU must file the petition for K-1 fiancé visa for him to enter US. You must prove you earn enough to support self, any dependents, plus him. IF approved, you have 90 days from the date of his entry to marry & file for his Adjustment of Status. No alternative if the two of you are planning to live together as husband & wife in the US. And if you divorce, you remain liable for his full support for at least 10 years. The only way out is an annulment based on proving he deceived & defrauded you for monetary & immigration benefits, and you have to get him back to his own country.
Tell him you want him to sponsor your immigration to his country, that you want to get to know his family, friends, country & culture. If he balks, he's using you.
If you have concerns about being used, you probably are. Listen to your gut! And watch for any of the red flags common to marriage fraud, immigration fraud. Hasty declarations of "love" in an online communication, when you two have not yet met in person. Insistence on moving to US & refusal to consider sponsoring you to his country. Wanting money from you, especially prior to establishing a strong commitment to the relationship, or "pleas of poverty," inability to find a job in his own country (the person likely has no skills to work & contribute to a married household), and similar excuses.
Since he does not get "citizenship" through marriage, but immigration & possible legal permanent residency through your sponsorship, what he says about the "marriage contract" is totally irrelevant, and he could be playing you. Assume he knows such a "contract" or prenuptial agreement is not relevant or enforceable! Drop the "marriage contract" nonsense as it does nothing at all to protect you.