No one on this site is going to be able to assure you that he will have no problems, and that he will be allowed in for 6 full months. The burden is on the two of you to be prepared to prove your case if he gets questioned. Currently most tourists receive a 6 month stamp when they enter the country without a problem. However, as I stated above, no one is entitled to this, and there is no way to guarantee that you will receive this when you go through immigration at the airport.
Coming in for a long period of time arouses suspicion. The first way to alleviate this is to have a return ticket. Your boyfriend should also be prepared to show that he has plenty of money to support himself during his trip. He should also not lie under any circumstances to immigration. Don't volunteer information, but if asked who he is visiting, he should be honest. There is nothing wrong with coming to visit a boyfriend/girlfriend/fiance, but it will arouse suspicion, and once again, you must find ways to mitigate this.
If you are very concerned, you may want to look into applying for a visitor's visa at a US consulate. I'm not sure what the going rate is for those.