We found the best way was to prepare a scrap book with pictures of him and you and a house and maybe some drawings of the house and things that he like to do and go through the scrap book with him. Start introducing the word adoption in a sentence. Believe me it will be harder when he is older, I think now is a perfect time he understands enough. Do not make a big deal out of it though.
Reassure him and tell him how much you love him and ( not knowing the circumstances of his particular situation ) I would tell him something like his mummy that had him in her tummy loved him very much but she was unable to look after him and that she wanted him to have a good home etc and that she chose you for this job. Keep reassuring him this and just by talking opening about it he will be OK. I have done it twice and it works although I may have started to introduce the word adoption from a slightly earlier age.
Please keep us informed of your progress with this as it is really important that he knows how much you love him.