My teacher says we should always keep our weight on the external side of our feet

I've recently started taking tai chi lessons (yang style), my teacher almost never lets his big toe touch the ground, and he asks us to always keep our weight on the outside edge of our feet and on the forefoot.

My feet and calves hurt when I do it for a while (like in some qigong exercises), but he's right that it prevents my knees and ankles from collapsing inward, which is my main postural problem.

I checked on the internet however, and nowhere I could find this same advice. I'm wondering whether this is regular tai chi or he's drifting a bit from the standard teachings