If he has permanent residency in Italy then he can apply for entry permits directly with the Hong Kong Immigration Department. An Exit-Entry permit is not required, as those are only issued to current mainland residents.
For procedures, see https://www.immd.gov.hk/eng/services/visas/overseas-chinese-entry-arrangement.html
You can sponsor him for visit visas which could allow him to stay for up to 90 days at one time.
However it's not likely he'll be eligible to live with you in Hong Kong on a long-term basis. Depending on your situation, you may want to consider a cross order living arrangement where he lives in Shenzhen or other nearby areas, at which time he will become a mainland resident, and may need to apply for an Exit-Entry permit (but recognized overseas Chinese often has different rules/procedures).