Next week I will be meeting my future stepson for the first time, and I desperately need advice!?

My fiance has a 3 and a half year old son who I will be meeting for the first time next week. I am so nervous, I just want everything to go well. My fiance and I have been together for over a year, and the entire time he has done nothing but struggle with his ex. She filed a restraining order when they broke up, for no reason, other than to gain full custody. After the year order was up she filed through the court for full custody but dropped it a few weeks ago after months of waiting and fighting. Now she is moving back to Michigan to be with her new boyfriend, who happens to live here in Michigan as we do. All of the sudden her boyfriend has convinced her to have a change of mind and is willing to let my fiance finally see his son. We (me, my fiance, his son, the mother, and new bf) will all be meeting up in a public setting, to 'reintroduce' my future step son to my fiance, since he might not remember him well. Keep in mind the mother still hates my fiance and me a great deal, she is only doing this because her new bf is understanding on my fiances part. My question is, has anyone else gone through this? How should I act in front of his mother, as to not anger her angry or make her feel like im not intruding? Would it be inappropriate to hug him? Should I stay silent and let only my fiance be involved and wait til we have him alone to try to bond with him and start a relationship? This first meeting is critical, and im just so scared and nervous! Please, and advice would help!