"I have a client who is a 40% shareholder and pays for a lot of on the job expenses himself without reimbursement."

The S Corp should establish an Accountable Plan, so that all employees are reimbursed under that plan, for validated business costs.

Thanks.  I understand the accountable plan should probably put in place, but since it's not is there an option for the shareholder to deduct the expenses against his k-1 income?

Your taxpayer might be able to "go back in time" and reknit this sweater a bit.

This article covers what sorts of ordinary business expenses are covered for reimbursement and also covers some methods for timeliness:

https://www.journalofaccountancy.com/issues/2020/feb/employee-expenses-accountable-plan.html

When you have direct "job related" as in client project costs, I would recommend they figure out if the business knew he was buying out-of-pocket, such as, "I'll buy the Home Depot order for the Smith's kitchen on my credit card so I can get the mileage." That might fall under loan from shareholder, and/or that timeliness article whereby the business knew he was making each purchase, personally, and he gave them proof (perhaps for job costing or billing to the customer) and then the annual summary might qualify, even though this is April. Of course, as my dead father used to say, documents are self-documenting, so you date things as they apply.