It sounds like you are taking this very rationally despite your jealousy, and handling it wonderfully so far. I would say the main thing to remember is communication, talk about everything as much as you can, and don't hide any feelings or thoughts no matter how silly or unimportant you think they may be. Communication is the key to making any relationship work, especially a poly one.
As far as your fear of him falling in love with another guy and leaving you, try thinking of it this way. He was a wonderful girlfriend who he loves very much and who makes him very happy. His girlfriend is trying to accept him being with another man, because she loves him so much that shes willing to work through her jealousy for him, even knowing it might be rough. If he falls in love with another man, why would he throw all that away? He has a chance at having both, so why choose just one? I know its hard to really understand from a monogomous perspective, just remember that no matter if he falls in love with someone else, his feelings for you will remain just as strong, if not stronger because of how happy will be that you were open to this. Trust me, there have been times i have been forced to choose between two partners because one was causing drama and risking our sexual safety with cheating and lying. Even then, it rips your heart out to have to leave one person you love for the other, its like a parent having to choose only one of two children. Just continue to remind yourself that his love for you is still there and he has no reason to leave someone he loves.
It is a good thing to discuss NRE or new relationship energy. Its very easy to get swept up in the new shininess of a relationship and neglect other partners. If he is aware these feelings will happen, he can take steps to make sure that especially at the beginning of their relationship he can shower you with love and affection and make sure you are getting the attention you need.
Also, at the start, you should make sure to discuss your boundaries with him. He is his own person and has the freedom to do whatever he wants, but you should outline what you are comfortable with and what the results will be if those boundaries are crossed. Personally, I would suggest something involving STD testing for the new partner so that you aren't put at risk. Also, managing time and making sure he spends at least an equal amount of time with you, especially at the start when the new relationship will be so exciting, is a good thing to discuss.
I hope this all helps, and I'm sure as things progress you'll get a better idea of what you are just fine with and what you need to work on some. Don't worry, if you set your mind to it you can conquer your jealousy and become more confident in your relationship, which even if this venture into poly doesnt work, is still a good result.