Select a more suitable gear ratio with him. I assume he has a triple so start small and work up to a more suitable gear
for him. Work with the mid ring and the 19 or 20 cog and adjust to where he is comfortable and can maintain for a while. If he gets discouraged remind him "That is why the experts invented all those gear combos..."
And like the Warden said, if after a while there is no improvement, then worry...
Also, be careful not to discourage him by suggesting a tougher ratio, this may turn a training ride for you into a pretty slow and dull ride but keep in mind that he will get there after a while. It is better
not to turn him off to cycling and subsequently make him regret the bike investment.
And tell him to quit. Best thing I ever did...
Good luck!