It might also help to find out all you can about what was going on in the community. Was there an outbreak of an epidemic, in which his parents might have died? What religious group did his adopting parents belong to? There wasn't Social Services back then, so his adopting parents had to find out about him through their social circles. Did his adopting parents have other children, leading one to think that they felt extra compassion for him? Or were they childless, leading one to think that perhaps they were looking for a child to adopt? I'd say try to get as much info about the context as possible -- you never know when the clue will turn up or the penny will drop!
The Adoption Angels have techniques that can help you, also.