Are both families going or are just taking the friend yourself? If both families are going, maybe meet at the airport and act surprised they are there too and ask if they would like to go to Disney together. If you are meeting up earlier, maybe have the friend walk in with a Mickey balloon and surprise your son? Look on YouTube, lots of cute ideas on there.
My parents surprised my sister and I one year with a scavenger hunt around our house, and in the end we came across the magicbands and bags for the parks. You could always do something like that the morning of!