Alex was a relative of Heather McCrimmon.
He celebrated Christmas at Granny McCrimmon's house along with Heather and the Tenth Doctor. During the celebration he stood next to Heather and her dad. (COMIC: A Merry Little Christmas)
Behind the Scenes
It wasn't mentioned in the Comic what kind of family relation Alex had to Heather. Since he stood so close to her and her dad he might be Heather's brother.