You could file an I-130 petition for him now. He would be in the F2B category which currently has a wait of 6.5 years. If you become a citizen he would move into the F1 category which has about the same wait (actually currently it's a little slower than F2B, and you can choose to opt out of converting F1 if it's advantageous). He cannot get married before you become a citizen, or his petition will be dead. (If he marries after you become a citizen, he would move into the F3 category, which has a wait of almost 12 years, so it's best he not get married at all to avoid a long wait.)