For a more gradual solution, take them back to their own bed, stay with them for a short time, slowly decreasing the amount of time that you stay.
A more unusual approach is to wake him during the night before he normally wakes himself. Give him a drink, settle him down again and hopefully he'll stay there. Gradually stop waking him and you might break the habit.
Professor John Pearce, author of the Baby And Toddler Sleep Programme (Vermilion, £7.99)