Remain in Featherweight and get 3 or 4 wins over the next 2 years, then challenge Volkanovski for the title again. He will be able to make some okay money and get relatively easy wins at Featherweight for the next two years, even if he loses to Volkanovski again when he gets his third title shot.
If he moves to Lightweight, he will not stand a chance against the top 6 fighters of the division (and this includes Khabib, who is a million times worse than Volkanovski), and he sure as hell won't make a good living barely winning against lower ranked Lightweight fighters (who are all still risky, by the way).