I don't agree with threats but I would have been damn angry too. Look at it from his view. He helps you get started by providing the money needed to go out and get customers of your own. You finally have enough customers to not have to sub-contract anymore so you tell him you are going it alone and the next thing he sees is his customers leaving for you.
In my humble opinion the one or two customers that want you to take on servicing their lawns was not worth burning the bridge. You were better off with the industry contact instead of the couple of customers that are likely to to do to you what they did to the man that was kind enough to help you get started.
No way would I ever take a customer from our sub-contracter and although the goal is to get to the point where we don't need to sub-contract so much I STILL wouldn't even take on a customer that has cancelled with him without talking to him first and making sure he is ok with it.
It is this very reason LCO's are hesitant to trust sub-contractors.
If you had been as loyal as he then you would have told him his customers were dis-satisfied way before it got to this point....like I said what comes around goes around. Good luck with your new customers.