Visa quesiton to come USA

I am a US citizen and my mother has green card. She is in China right now.
My step-father does not have green card.

I want to invite them to come here to visit us for a couple of months.

What visa should my step-father apply? Immigration or non-immigration?

I would like him to apply B1/2 tour visa but don't know if there is any problem since my mother has green card.

Thanks,

Xiong