You can't be common law married to him, while he's still married to someone else. No, you have no legal protection in your relationship. His wife, has all the legal rights of a wife, and any money you put into ANYTHING you and he do together, could become an asset that his wife can claim half of (so you'd best keep documentation of EVERY penny you spend toward your life with him, so you'll have some little chance of not losing everything if they don't divorce or if they reconcile).
If they've been legally separated 5 yrs, he could probably have stopped paying alimony YEARS ago. She's benefiting from them remaining married, so why would she push for divorce. He claims he doesn't want to lose half his pension, but does he intend to pay her alimony until she dies, and which is going to cost him more in the long run? He also has rights to her assets to include her pensions and such.