Hi potatoaddict
Our son was three when he came to us.
Others have given advice I would add....
We sent ahead a talking book of photos, a dvd (made by our dd with my dh's help) and a blanket for him with his name on (which he loved)
We started out calling ourselves mummy and daddy from day one or two and he followed suit
We took our lead from fc
We took a new teddy, a toy for him on our first meeting and when we left him we gave the teddy and asked him to look after it until we saw him the next day
Over the following days we took him out anywhere we could go, any kind of place you think he might like, a farm, a park, a garden centre, a place with animals, and even a soft play at a quiet time was OK (it was almost deserted) but lots of people will say avoid soft play!
Each time we visited we tried to have a certain bit of structure and do a story with him once or twice, we let him turn the pages and as well as reading the story we asked him questions (can you find the bird in the sky etc?) just so it was iterative.
We took puzzles and had them in the car in case needed.
We asked what food to bring and took his favourite biscuits etc.
Happy days
Six months ago!
Best of luck.