Hi Bonnie
The parents will make an introductions book with pictures and little captions, sometimes they get little books which can record voice as well, and also sometimes parents are told to create a short DVD for the child to watch of them. So you should have things to go through with him, so he is introduced to his new mum and dad before meeting them and he will know what they look like
You should call them his new mummy and daddy from the off, that's what he will call them given his age. Older children can be different (my DD1 calls me by my first name, and DD2 started off as 'mummy lilka' then dropped my first name later on). His introductions book will probably be captionned 'mummy' and 'daddy' under pictures of them.
My DS was only told about me a few days before he met me. Given his age, it would have been confusing to be told he had a new mummy, and then she didn't show up for days on end. A week is a year to a 2 year old! He was told he was getting a new mummy to look after him forever, and look here's her picture and her house, and so on. Obviously understanding very limited at that age.
The move will be traumatic of course, but with my DS he didn't grieve until he had been home a week or so, when he realised he wasn't going back again. He didn't really understand what was happening when intros were going on (he was 23 months old though so a bit younger).
The best way (IMHO only) is to encourage him to play with them as much as possible and interact with them. The first day with DS I went for an hour and a half, and I only talked to him and played with him. His FC's were great, they were in the room so he felt safe, but on the sofa slightly away from us. I did more and more after that - I came to do the bedtime routine several evenings and give him his bath. I took him out a few times to the park, and later introduced DD1 and DD2 and took him to visit home. We moved his things over day by day, so it wasn't hard packing all his stuff at the last minute and fitting it all in the car!
It can be very nervous at FC's house, because obviously I was there quite a lot at odd times of the day so I could put him to bed, and give him breakfast a couple of days. I felt like an intruder tbh! She said 'just us the loo whenever, treat it as home' but I had to ask every time, I just couldn't treat her house like home!
Hopefully you'll be able to meet his mum and dad beforehand. I had a lot of questions because the information I got was a bit lacking, and I wanted to keep his routines the same so not everything was changed at once
Hope that helps, ask any other questions you need to!