How do I get a Czech Republic residency permit for partner?

Hi I'm a US Citizen. I was offered a job in the Czech republic. I contacted the Czech embassy about residency for my partner and was told he can only stay for 90 days because we are not married. According to the embassy, family reunification rules only apply to married couples and their children. I contacted a law firm I found online but they want 150 euros just to answer a question, and more to work on the case. What does it mean - he can only stay 90 days? Can he leave and come back? He gets a tourist VISA for 90 days, he leaves the country, comes back and gets another one? the embassy won't answer this question Does any one have any experience with this? or know where I can find information? thanks