father owns business, son over age 21 only employee

Have a company , father owns 100% of business and son over age 21 is only employee- I realize due to attrition rules the son also owns 100% - so is this a one man plan that I can file a 5500 EZ , or do I need to do an Sf?

If it is an S-corp, yes.

If DB/CB plan, PBGC may come into play, depending on the biz.

it is a 401k plan, and s corp

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Have a company, father owns 100% of business and son over age 21 is only employee- I realize due to attrition rules the son also owns 100% - so is this a one-man plan that I can file a 5500 EZ, or do I need to do an Sf?

Careful readers will observe that the original post included 2 questions, so it's unclear which one received a reply.

5500EZ instructions state that for a family biz filing as an S-corp, a plan covering only family members, can file an EZ. Owner, spouse and their children.

Consider warning one’s client that a Form 5500-EZ classification does not change whether ERISA’s title I governs the plan.

An ERISA rule treats a proprietor’s or partner’s spouse as not an employee, but does not provide that interpretation regarding a proprietor’s or partner’s child.

29 C.F.R. § 2510.3-3(c) https://www.ecfr.gov/current/title-29/part-2510/section-2510.3-3#p-2510.3-3(c).