Why do you want them to persevere? They need a good reason, and they need help taking little steps towards a big goal. Find something really appealing, like saving up money to go on a big trip, and then check in with them periodically to see how their saving is going, and what they struggle with. What moments are their weakest and how to support those.
Like others have mentioned, strong motivators for us are pleasing those we love, curiosity, and self-growth. Much of the struggle is in understanding why sticking to something is more preferable than abandoning it to explore a temporarily more satisfying avenue, and how to keep one's positivity when voluntarily abandoning the joyful option for the perseverant one.
A strong disciplinary structure (with reward for good behavior and removal of privilege for bad behavior) would help teach discipline.
You affirm his inner self and personal competence while helping him maintain an external structure for attaining his goals. We have a really hard time creating our own external structures and sticking to them. The more demonstration and hand-holding through that, the better.How do I help him put forth effort when things are difficult without making him feel like a loser?