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, Alex 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). According to another account, Susan and David had three adopted children, named Ian, Barbara and David Campbell Junior. (EDA: 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. The Doctor wanted Alex to have an upbringing on Gallifrey to fulfil his full academic potential, however Alex saw Earth as his true home and decided to stay with his mother. (BFA: An Earthly Child)

Six months later, Alex had christmas dinner in the TARDIS with his mother, the Doctor and Lucie Miller. After which, Alex took up travelling the world with Lucie. (BFA: Relative Dimensions)