Mom of five year old concerned about change in sons behaviour when boyfriend is around

Hi Sara, I have been dating my boyfriend for a little over 2 years now. During the past year my five year old son and my boyfriend have been hanging out at least once a week, no sleepovers yet. My son likes my boyfriend but whenever he is around my son misbehaves - he whines, cries and just isn't himself.My boyfriend wants us to move in with him eventually but if their relationship doesn't improve I dont see it happening. How can I get my son to behave appropriately around my boyfriend? I think my son views him as a friend and someone to play with. Your thoughts?