In economics we have supply and demand. As houses become more expensive the demand for the remaining affordable houses increases. It is natural for him to be opposed to affordable housing if he owns some houses, because in a capitalist world he expects to profit off their sale.

You have to counteract this increasing demand by increasing the supply of houses: build more houses! Not condos or apartments, because that’s a different market. But building more houses is necessary, and a charitable organization could really put its donation dollars to good use by doing that.

When supply increases, prices drop as long as they increase faster than demand.