Get fewer of his calories from animal products, for one thing - not necessarily go vegetarian/vegan, but cutting down by 50-75%, and switching to grass-fed, o

Get fewer of his calories from animal products, for one thing - not necessarily go vegetarian/vegan, but cutting down by 50-75%, and switching to grass-fed, organic meat (which doesn't require petrochemical derived fertiliser in its production) makes a significant difference. But be mindful that not all plant calories are equal, especially those that are air freighted (more of a problem in NW Europe which airfreights a lot of fresh fruit and veg in from African countries than the US). Take trains not planes wherever possible, and avoid long haul flights entirely if you can: one return long-haul flight can undo all the good of a year's worth of biking.