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.
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What are the requirements to getting a permanent residency by investment in Japan?
My youngest son is 25 with special needs so we can't be separated. Can I bring him to Japan on the same application? How much do I need to invest?