How do deal with this situation?

[domenikbrenner]

I have successfully finished a project for a client and I initially said that I'd create an executable file for him that he can run daily. It's a webautomation, relatively simple. 

During the project we communicated a lot and I kept sending him example outputs and made adjustments as he needed them, and when I was done, I sent him the exe file with an example output and didn't think much of it any more, as it works as he requested (I know because we were in touch while I was doing it and I got constant feedback until he told me it's exactly how he wants it.).

Anyway. The job was automatically accepted, and right after he sent me a message saying "Hey Dom, I just had time to look at it and I really want the source code so I can make changes in the future if necessary".

I said he'll get an executable file he can run with windows task planner every day before I even sent him the custom offer, and now, after the delivery is done, he wants the source code. But I don't want to just give him the source code. If he needs any changes on this script, he can come back to me. If he really wants the source code, it would have been more expensive. 

What is the best way to deal with this now? He clearly didn't read my leading message and custom offer properly, but I also don't want to just give it away for free now. 
How do I word politely?

I want to say "I told you it's an executable (Here is the screenshot). Now if you want changes, you can come to me or you can pay extra to get the source code"

Edited September 17 by domenikbrenner