How to stay in touch with professor after graduating?

Disclaimer: I asked this question on academia since it is very much so a student/professor relationship despite both him and me being significantly more industry oriented.

About Me: I recently graduated from a degree (4-5 months ago) and am working in industry. I'm 20 years old.

Dilemma: I recently found out from a friend of mine that my lecturer at EIT and my supervisor/mentor throughout my degree was planning to develop a new paper. I sent him an email telling him how the paper sounds like it would be very useful because as a graduate I found X, Y, Z to be very critical in industry. I also mentioned my new job in the email and some other stuff about the paper.

Anyway he replied saying

Good to hear from you. Great that Unison has offered you a job that appreciates your skills; at least, that's what I understand.

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I hope you like your current job, and that you keep challenging yourself. We should stay in touch. Cheers,

So my problem is, what does "staying in touch" mean? How do I properly stay in touch with him?

Other Questions:

The first answer to this question had the best answer I could find, however It did not help me.

The easiest way is to keep working on projects with them. If that ship has sailed, then the next best way is to be friends with them on social media, which is to say, be friends with them in real life. Barring that, you have to work to communicate with them on a regular basis about topics relevant to your shared interests: go to conferences and strike up conversations with them, share interesting papers with them via email (i.e., "Did you see this new paper from prof X? What'd you think?"), propose joint projects and write joint grant proposals, etc. It's not hard, but it does take active work

1. I'm 20 and he is around 40+ and leads a very busy life (kids, own business on the side and etc), so I'm not sure the whole "friend" thing would work overly that well.


2. I'm in industry, he is in academia so projects/proposals are not really viable. (however he is more teaching/interfaceWithIndustry sort of thing as I do not believe he does any research any more). Also, "side-projects" are a viable option as we are software developers, however I have my plateful with them already and he has his own company as a side project, so there is no time for them really.

Another Related Question: How does one maintain academic contacts?