Hard to say for us on what would be good to get them because, well, we don't know them or what they are like. The fact that they booked you a place to stay is really going out of there way. As for meeting his mom and him, dress in something comfortable and you like while at the same time pressentable. I had to do this with my ex before my self.
If your only worries are about him not likeing you well thats a commen fear we all have on the first time we meet some one so try not to think about it so much. I say try because your going to any way, but having it at the back of your mind only will make it less stressfull.
Wish you the best of luck, fingers and toes are crossed for you.