Mention the topic or project. Don’t say anything about it, this is just the raising of the subject. Like “Hey I know you’ve been interested in such and such.” Then stop, let them process not just the topic but also their feelings about it. Maybe they’ll say something, maybe they won’t. Count to three. Be sensitive to the possibility that you might be raising their hackles, getting them in a position of readiness to be defensive. We’re going to try to walk through this without getting them hostile.
Ask, What's the status? Use the words, “How’s it going?” This puts the onus of judgment off of your own shoulders and allows him to be honest with his own feelings before he even talks about it. Then let him talk until he’s done. Your curiosity is precious.
Ask him to explain the current hurdle or blockage that is stopping him.
Ask him to explain the details of the next, first step.
Express the value of the project or the outcome in terms of the achievement, its significance and meaning, the positive emotions that will come out for himself and for others. The relief, the accomplishment, the doors it may open. Its tie-in to the long-term. Why exactly it will be so cool.
Do give him a place to speak. Don't speak for him, it takes away his agency. That's the whole point, to put him into an emotional state of agency, of forward movement, to encourage his agency.
There you go. Let me know if it helps.