Seriously? Why on earth are you doing this?
You have handed in your notice.
I guess it may not be unreasonable of him to ask you, as part of your current duties, to spend work time on recruiting your successor - but that is the extent of it. His choice, his expense, his workplace.
It is NOT your job to find your (or 3 potential) replacements or face threats!!
He is STILL just out to make life difficult for you.
Your notice period is your notice period. However, he rather trampled over that by trying to stop you working elsewhere while working your notice with him - so ignore him.
Reiterate you are leaving.
There is NO policy about second jobs so it is NONE of his business how you spend your time outside of work.
Work your notice and start new job, DO NOT TELL HIM - do NOT engage in that conversation.
With the best will in the world, you are perpetuating the situation by engaging with him.
If you are taking a step up in your job / career, you will need to start to learn how to respond professionally in more challenging situations - practice now!
He has had time to plan for your replacement - in fact he probably has started his own recruitment search.
Sorry, this is longer than I imagined it would be - basically DO NOT engage with this farce. You can work both jobs for the last 10 days or leave early END OF.
I would never normally dream of suggesting calling in sick, but on this one instance...
Have references etc been sorted with your new job?
If not you just need to outline briefly that you boss doesn't want you to leave... etc etc...
Provide some good personal references to them.
DO NOT SIT IN A MEETING WITH HIM HAVING A ROW.
They do not have you over a barrel. You have a small part time job there - just leave. He is just playing with you.