latest date to contribute to retroactive solo 401k

sole proprietor has not filed his tax returns yet (on extension).

wants to adopt solo 401k retroactively, and make maximum deferral and profit sharing contributions.

What is the latest date he can adopt and contribute? 

I'm being told that under Secure 2.0 he cannot defer after April 15. Is this true? (Getting conflicting answers.)

your help will be most appreciated!

The sole proprietor is in luck!  SECURE 2.0 provides for retroactive first year elective deferrals for sole proprietors (sec. 317 of the Act and sec. 401(b) of the Code).

Specifically, 401(b)(2) reads:

If an employer adopts a stock bonus, pension, profit-sharing, or annuity plan after the close of a taxable year but before the time prescribed by law for filing the return of the employer for the taxable year (including extensions thereof), the employer may elect to treat the plan as having been adopted as of the last day of the taxable year. In the case of an individual who owns the entire interest in an unincorporated trade or business, and who is the only employee of such trade or business, any elective deferrals (as defined in section 402(g)(3)) under a qualified cash or deferred arrangement to which the preceding sentence applies, which are made by such individual before the time for filing the return of such individual for the taxable year (determined without regard to any extensions) ending after or with the end of the plan's first plan year, shall be treated as having been made before the end of such first plan year.

Just make sure the business is unincorporated, the sole proprietor owns the entire business, and there are no employees.

Note that the ability to take advantage of this provision is limited, and the provision in the narrow set of circumstances supersedes prior provisions that required all deferral elections to be made before the end of the plan year.  This easily could result in getting conflicting answers.

SECURE 2.0, Section 317, specifies that it is the time for filing the return of such individual for the taxable year..."(determined without regard to any extensions)"...

Caveat - I did not check to see if any subsequent guidance modifies this.

I see Paul already replied. My response was with regard to deferrals only.

Belgarath is right - can still do PS, but deferral time ended 4-15.