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. 

He replied saying 


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



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. 
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?
As far as my experience goes, this 'we should stay in touch' with no concrete suggestion as to how you should stay in touch, means he does not expect you to actually stay in touch on an ongoing fashion. 

The way I interpret the meaning of this phrase as it appears in the email you quoted, it means roughly the following: "I am a busy man, and I have no particular thing in mind that we should cooperate about at the moment. But I like you, I am thankful for your comments, and I am impressed with what you achieved so far and by your potential. So if at any point in the future you see a concrete way in which our relationship can be put to use please contact me, and I will do the same on my side."

So I would just answer back saying something like "I'm glad were helpful. I'd be happy to chat more if you ever want [more comments about the current paper or anything related to the it or the job that you are doing now]."