Alex Campbell

Alex Campbell was the son of Susan Foreman and great-grandson of the Doctor

Early life
The only (biological) child of Susan and David Campbell and was born around 10 years after the events of The Dalek Invasion of Earth. Susan and David decided not to tell Alex aboout his true heritage as they believed it would make it hard for him in his life on Earth. (BFA: An Earthly Child). He may have had three older, adopted siblings, David Campbell Jr, Ian Campbell and Barbara Campbell, presumably named after Davi, and Susan's former travelling companions, Ian Chesterton and Barbara Wright. (Legacy of the Daleks)

Meeting With The Doctor
After the death of his father, Alex became rebellious and joined various youth groups opposed to the new Earth Council, when Alex was 17 he met his great-grandfather who saved him from the Guldreasi and told him of his true origin.