The main point he makes is that, regardless of how inexhaustable our sources of energy are, increases in our rate of energy use are still necessarily limited.
He notes:
1. Economic growth is always accompanied by increase in energy consumption.
2. Nearly all energy consumption (whether that be running current through computer chips, burning fuel in combustion engines, or just metabolizing food) has the effect of heating the surrounding land, air, or water.
3. If humanity's rate of energy use increases 2.3% every year, then the associated heat dissipation will be enough to cook us all within about 4 centuries.
4. Economic growth without an increased rate of energy use is not obviously impossible a priori, but it's definitely hard to imagine.
[1] https://dothemath.ucsd.edu/2012/04/economist-meets-physicist...