Aaron Shapiro's firm cleared a regulatory hurdle in December, when the IRS determined that companies can let employees benefit from Carver Edison's loans without jeopardizing the tax-exempt status of their ESPPs. (Photo: Bloomberg)

(Bloomberg) –Aaron Shapiro's quest to rethink employee stock purchase plans began with a startling insight: If only his mother had participated in one, she'd be a millionaire today.

A few years ago, she asked her son for financial advice and mentioned her employer's ESPP, which she didn't understand. So Shapiro, 26, who got his finance degree from Babson College, dug into the prospectus.

The employer, UnitedHealth Group Inc., allows workers to buy shares twice a year at a 15 percent discount. There's no minimum holding period, meaning participants can sell the shares immediately and pocket the difference.