I'm going to take a different tactic from the answers here and say you should be wholly truthful (and not give a non-answer like "I found a different/better opportunity"). But if you want to go this route, be very careful with how you word your conversation.
The reason I say this is because you said you and your boss are close and you would like to remain that way, and your boss is the CEO and founder of the company. If you care about him, you should want his business to succeed, and part of the business succeeding is being able to retain good people. Therefore, you should feel, personally (and not professionally), that you have the responsibility to tell him as much of the truth as you can to help him not implode his business.
However, once again, it's very important how you say it. Here are a few tips:
You don't want to assign blame; rather than saying "You did X and I hate it", you can say "I noticed X was occurring, and I found it uncomfortable because Y. You may want to reconsider X". Focus on the problems, not the people.
Be as specific as possible about the things you don't like; for example, you mention the company culture. Rather than saying "the company culture has changed and I don't like it", say something like "When I joined the company, we used to have a lot of parties and company outings, now all we do is work, work, work, and I don't feel as connected to my coworkers; I valued that experience and I don't have it anymore", or perhaps the inverse: "When I joined the company, I could just work and then go home in the evening, now I feel obligated to attend all sorts of company events after work that I'm not comfortable with".
If you are refuted, don't take it personally or start an argument. If you say "I don't like X because Y" and your boss says "I think X is a good idea because Z", just drop it. Don't try to argue, it's not your place. You made your point, your point was not accepted, move on, don't argue. It's about providing feedback and trying to help, not being right and making him do things your way. At the end of the day, he's the CEO, not you.
It's not conducive to a successful business to receive no feedback, just as it's not conducive to a successful interview to receive no feedback (yet companies do this all the time). If you don't care about the business or the people in it, then you can give a non-answer and just leave on as amicable terms as possible as other answers suggest, but if you actually do care and want to help make things better, and you have that sort of relationship with the CEO where you can make those suggestions without hurt feelings, then I say go ahead and do it.
For me the problem is on one hand I do not want to offend my boss. He is nice and we have good rapport going but on the other hand I do not think I can be productive in this company ever.
– Unknown Unknown Aug 15 '22 at 17:32