Varley, from Blackpool, told police Preston had accidentally drowned in a bath, but a post-mortem examination discovered the child had suffered 40 injuries.
His partner, John McGowan-Fazakerley, 32, has been found guilty of s3xual assault, child cruelty and allowing the d3ath of a child.
The pair will be sentenced on Thursday.
Preston was adopted by Varley and McGowan-Fazakerley, a financial sales manager, in April 2023 and in the next four months he suffered sexual abuse and ill-treatment, the trial at Preston Crown Court heard.
Preston was born in June 2022 and taken into care by Oldham Council.
