Working for a previous employer

oh_really

in_a_pickle_5 said:

My current employers won’t make it easy for me to leave...

You tender resignation, work notice period and walk out the door, though not to the employer you are asking about but to one without the personal baggage.

ACG

There could be ways around it. 
Does he own the new business or is it in the name of his wife/parents etc? 
Ultimately if the new boss is prepared to take the risk then it is on him. Ask him what happens to you if the previous employer finds out and kicks up a fuss. 

Jillanddy

prowla said:

If you set up as a contractor limited company then he wouldn't be employing you, but rather engaging the services of your company.

Not necessarily. Quite apart from the fact that such a strategy may not get around a legal contractual clause anyway, then that could simply create even more difficulties for both parties. Just because someone says it isn't an employment contract doesn't mean it isn't - such strategies to get around employment law are often used and abused, and often fail! And contractors also have few employment rights. And the employer could still be subject to being sued. 

The OP clearly believes that their current employer will make things as difficult as possible if they leave. So going back to the old employer where the new one has real leverage seems foolish. Perhaps time for a clean break and find a new employer entirely?