Does this guy have an actual product? Is the product a stand alone product such as a toy or item that can be sold as an end product to the consumer? Or is it a product that is part of a final product that is produced by your potential clients? Is it even physical at all or is it a process? Is it patented?
Has he formed a company around the invention? If so, does he have business partners? Investors? If the answer is yes- do they all share the same vision? Do any of them HAVE a vision?
Again, regarding partners (if any) - who contributed cash, services, both? Of those that contribute cash, who makes the decisions on what gets done when, where, how and by whom? Was/is this guy counting on any of the partners to market the invention/product? Are all of the people involved holding up their end of the bargain?
I ask these questions because sometimes there are things going on behind the scenes that might give one the impression someone is flying by the seat of their pants, when in fact they are flying with a high drag coefficient.
Talk to the guy. Ask questions. Find out what’s up. You may be surprised.
What is your desired outcome regarding the product/company in question? Long term involvement or build it, grow it, and sell it off? How many years on the patent, if he has one? Any follow on IP that could add value to a potential buyer?
You say he could probably market the product better than you can- what exactly do you mean? Is he a “closer”? Again, maybe he has a solid vision/direction/plan but no chops when it comes to closing a sale - and knows it. Maybe he has an aversion to the typical small talk chit chat BS and prefers to work behind the scenes.
I know I didn’t actually answer your question- but hopefully you can find out the answers to some of the above and the solution will become clearer- or at least less opaque.