Can my employer ask me to sign a contract stating that I cannot sue him for any reason? Even if he violates acts such as the Civil Rights Act?

Your former employer cannot "make" you sign this, but if you don't, you can't "make" him give you the bird.  Your employer is asking for a release, which is common, although there are some statutes in California that impose certain requirements which must be met for a release to be valid in an employment context, and it doesn't sound as if those requirements have been met here.  I would think that your main concern would be whether you believe that you have a valid claim against your former employer (I am assuming that you are only being asked to release existing claims, and not claims which may arise in the future; such releases can be valid, but are much trickier and less likely to be enforced).  If you don't believe you have a valid claim, it would seem to be a minor risk - that you may be giving up an existing claim of which you are unaware - to sign the release.