The boy, who was 14 when the alleged attacks started, said he was abused by Leslie Paul, of Dulwich, south-east London, for more than a year.
He said Mr Paul, whom he met when he was put into a local authority care and assessment centre, also took a pornographic video of him.
Mr Paul, 42, denies five charges of indecently assaulting the boy, one of indecency with another child, a charge of indecently assaulting a third boy, and indecently taking photographs of him while he was aged under 16.
The first teenager, now 18, said that soon after they met, Mr Paul, a residential social worker at the centre, began giving him presents.
On occasions he would visit the social worker at his home, but was warned not to tell other staff about it 'otherwise he would get into trouble'. The boy said he was first assaulted during a camping trip to the New Forest in Hampshire, organised by the centre. After that, it 'happened so many times, I can't even remember'.
The boy said: 'He also took rude pictures of me in the bath . . . and videoed me rolling around naked on the bed . . . He offered me pounds 4,000 to pounds 5,000 not to speak to police. But I had already.'
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