I have seen more than 10 or 15 cases of developers who were hired to develop and evolve a product and get "bored" after they learn the new tech that brought them to the company/project and use this as an excuse to not finish the project or keep running the product/company.
I'm not saying that keeping working with new tech or new projects is a bad thing, but it is usually a bad thing for companies to keep such people when what they need is someone to help the company grows and move forward.
I would suggest you to offer him a position where he could use his intelligence and tech skills to help solve real problems for the company and not only program. A few examples:
* Put him in touch with your operational people - If you an e-commerce that ships physical products, let him known and learn how logistics works and what pain points they have
* Make him participate in marketing/product growing meetings and let him help bring more money in
* Enable him to help other developers or fix major problems in the product - not technical problems by themselves, but real problems that slow down the development
If he doesn't want to help maybe he is not a keeper and should be better off doing consultancy/freelancing projects where usually there is not much responsibility once the project is finished.
I see great value on being technically safe and capable, but most of the time what is most valuable is people eager to work and make things go forward for the company.