If my fiancé will enter USA as a tourist and we’ll get married here soon after, will it be a problem for him to get Green Card?

You cannot get married within 90 days of her entry to the US or else it looks like misrepresentation of intent at the time of entry and can impact on her ability to get a green card. Please let me know if you have any additional questions or how I can be of further assistance to you in this matter.

As long as you wait a couple of months before getting married, your fiance should be fine. I would also recommend that you do the marriage proposal after their entry into the US. That way they come with the intention to visit and then change their mind and stay. You should consult with an experienced immigration attorney. You will want to be prepared to answer questions a certain way at the green card interview.

If he enters the US with the intent to marry and immigrate, that could be a problem. If he comes as a tourist to just meet with you and then decides to marry, he can apply to adjust to lawful permanent resident without returning home first.

Every person coming to the US is presumed to come with intent to stay. Your fiance is coming on B2 visa for 6 months supposedly to see the Country, not to marry you. It's just might accidentally happen... Anyway, if he marries the citizen, this marriage will be investigated (when and if you petition) subject to fraud. You better have a good documentation to support your marriage against those fraud potential allegations. In addition, you would have to file a petition and be able to support this endeavor financially. If you do not have enough income and no sponsor to help you, your intention might be questionable. Having said all this, may be you will be better off consulting an immigration attorney.