An investment itself only will not bring you to a permanent residency in Japan. It would take at least one year after your stay in Japan with an HSP or High Skilled Professional visa status, otherwise a 10-year stay is required with another type of work visa status, including investor/business manager type visa. There is no specific type of visa status available for your son, while he has a chance to be granted a dependent visa under humanitarian grounds or a medical stay visa.
