does he have family he can trust in his home country ? maybe he has children but they sure will not go spend their money in their eyes, to a child that is not even his...
usufruct ends at dead and a lease cannot be inherited... so if OP dies, that is the end
unless the lease is to an adult he trusts here in TH ...
if he would manage to find a loophole, and the child is groomed as any narcissistic money obsessed adult, she will be guilted, at reaching adult status, so sadly give it up to the mother, her family, etc ... unless the OP can educate her financially and that this could be for HER future and not her undeserving mother, in the OP's eyes...