Can my boss threaten my Job if I became ill, and need time away from work? Can I protect myself from being fired for something else he comes up with?

Being on a disability leave does not guarantee that you will always be able to return to work.  In other words, your employer does not have to hold your job open forever.  The FMLA may cover your leave for 12 weeks.  You may have other guarantees of reinstatement in a union contract, individual contract, handbook, etc.  You need to speak with an employment lawyer about your rights and options.