Please do try to rekindle him.
Would trying to rekindle his lost spark qualify as not minding your own business? Of course it would.
Would it be a negative thing? Absolutely not. Let me explain.
Many times we are told to mind our own business, to not interfere with the choices of people etc. This is all true. But this approach to life also allows us not to get our hands dirty with closer relationships. Close relationships are filled with the responsibility of the consequence of our actions on the other person. If I see a friend acting in a strange way (for instance, he looks dull and sad), I can choose to intervene or not. The intervention comes with the risk of failing, of being wrong, of being perceived as nosy... The not-intervention, on the other hand, doesn't carry any risk, it's a disengagement. This is the safer option. But, in my opinion, relationships are built on the moments in which we decide to engage, to expose ourselves to risk, to go out of our way for the other person. What do you remember, that day when a person tiptoed around your pain or that another one in which a friend told you "Come on, you're not ok, now let's go grab a beer and you will tell me what's wrong"?
Sorry for this introductory paragraph, but I felt it was needed. Now I will step down from my soapbox and I'll give you some practical advice.
Rekindling the spark of a person is a difficult task. It can be broken down in some bits:
Make the person aware that he lost their spark.
Make the person aware of what caused the loss of his spark.
Help the person build a plan to rekindle it.
Help the person follow through the plan.
1. Make the person aware that he lost his spark.
In his "Tale of the Unknown Island", José Saramago says: "You have to leave the island in order to see the island". That is, no one recognises their current situation until they can step out of it. For this reason, I'd suggest you to engage in a long(ish) activity with him, preferably one that the two of you used to do with great enthusiasm. It can be a one-day hike, a short trip to a nearby city, a tour in the archaeological site in the city center... I'd recommend an outdoor activity. The farther from his everyday life, the better. If he's trapped in a dull routine, it will be beneficial for him. Moreover, the two of you can directly compare his current behaviour to the one he used to have in such situations (or his behavior in a nice activity vs the one in his routine) and recognise if it has significantly changed. If you see a very different behaviour and he seems unaware of it, you can speak up your concern.
Do you remember that time when we kayaked our way from Washington DC to Chicago? Without your energy, I'd have been stuck after ten minutes. How is it that you can't manage to cross the river, now?
If he makes up some excuse, push the subject some more. There is not an universal recipe about the amount of pushing to do: some people are very sensitive and fragile, so some subtle hints will do, while others are completely blind to reality, so you may want to opt for a more intense approach. Keep in mind that the realization itself may be a shock and that several activities of this kind might be necessary in order for him to recognise his novel dullness.
2. Make the person aware of what caused the loss of his spark.
This step can start the same day of #1, but it will take a lot of time to finish. It consists most of dialogue and active listening. It's probably going to be very long and intense for him. If you can trigger in him the desire of reflecting on himself and to investigate the root cause of things, it's already a great thing. It's already a small spark.
We don't know what the cause is right now, so I can't really suggest you anything specific. Maybe he even knows already the cause of his dullness (a family grievance? His girlfriend left him with his best friend?).
3. Help the person build a plan to rekindle it.
Now, what practical things he can do to improve his situation? Even better, what perspective on life can he have to not perceive his situation ad hopeless / depressing? What point of view did he gradually abandon during the past years?
4. Help the person follow through the plan.
When he has identified the approaches to follow to have a sparkling life again, your role is to just be there. Applaud his successes, acknowledge when he is full of energy...
Now some quick and general notes.
1. You cannot "save" him nor "fix" him.
You're not a knight nor a saviour and he's not a damsel in distress. That is, do not put yourself above him. You have your set of problems, he has his. You don't go and approach him because you're "healthy" and he's not. What is needed is empathy and not sympathy.
2. Your solution is wrong.
Even if you come up with the best solution ever, it will be inherently wrong, because so far it's only yours. He needs his solution, not yours. So in order for your solution to be effective, he has to accept and digest it. A good suggestion from you will come with some hint on how to digest it. That is, a good suggestion is always a personal one.