Originally Posted by
Kraco
No, it's absolutely the best thing not to tell. Telling the old man that would be the same as telling him that his only son is dead and a total stranger is controlling the son's body. It would be horrifying, no matter how level-headed someone is. Only an emotionless, loveless, and utterly uncaring person wouldn't mind. Like one of those typical isekai villain nobles, who only cares about his own ambitions and treats everyone like tools and trash, including their own children. Such a monster would actually be glad if their average son was replaced by a genius since it would raise the family's prestige and thus their own status.
What positive could come out of telling? It's not like the Japanese professor would ever be coming back. He's stuck in Farma's body for good, so it's now his only body. He has to live with it. The only price he has to pay by not telling is that his real soul (and Japanese name) won't be acknowledged, but that's a pretty insignificant price to pay compared to being able to live in a decent family, with the best imaginable resources available in that world to do what he loves to do: develop medicine and help sick people. Plus he now even again has a little sister; losing the sister was what drove him into medicine in the first place. If he told, he might become a homeless fugitive if he wasn't executed immediately.