Anyone have any tips on meeting boyfriend's children?? advice appreciated...

hi guys, so I've recently started dating a guy who I've been close to for about 2 years now, you know that kinda relationship where you first meet and feel like you've known them a whole lifetime?? yeahh that's me and my SO, we were destined to be together!! anyway, he's been married before, and has a son (5) and has practically adopted his ex-wifes son too (2) who has no contact at all with his biological father, so anyway, we've talked about me meeting his son but he wants to speak to his ex-wife first and then obviously speak with his son about me too...fair enough, what I'm asking is for any advice on how to handle things when we do meet?? I'm 27 and childless myself, although I do work with children I have no real sense of parenting as such, but I know that me and my SO are both in the relationship for the long haul, so I want to make sure I make a good first impression with his son and hopefully a lasting one...any advice would be much appreciated!! (and sorry for the essay!!) TIA, Emma