If it's like the US, you write him a letter. You invite him to come visit you, specify your full name, his full name and passport info, the dates, state that you will be responsible for his room and board (unless he books a hotel), his health insurance. In the US we can get special one-month health insurance for foreigners, for smth like $100.

Just ask him to find out what his government requires in the form of an official invitation - so you don't have to do all the work!

Удачи!