It's just not that simple. He might be right if he only has a few motors, all small, not running all of the time. 3 phase motors are not automatically more efficient just by being 3 phase alone, but it often works out that way. It's just because the motor mfrs spend more time and effort on making 3 phase motors more efficient as they tend to be used on continuous loads, whereas single phase are more often intermittent.
So if you pick out a NEW motor from a catalog, let's say 5HP 230V and look at the single phase and a 3 phase versions. The
3 phase will be a few percent more efficient., ie. 89.5% vs the
single phase at 86.5%. If your motor runs once per day for a couple of hours, the cost to bring in 3 phase just for that would never pay you back. Even if that motor runs 24/7 5 days a week, 50 weeks/year, that 3% of 6.7kW works out to around 1700kWH, so at $.15/kWh that's only $250 or so. If the PoCo charges him $5,000 to bring 3 phase into his farm, it would take him 20 years before he even paid for it, let alone see benefit.
But if he has 20 such motors, it's 1 year. NOW it makes sense.