A crown is not part of a root canal, it is a separate procedure and separate charge. The tooth is sealed with the post and filling, unless you break it or it falls out etc. You can opt NOT to have a crown and take your risks with either a temp or the post and filling. They are two separate things with two separate charges. Yes, it should have been done at the time of the root canal, as most people get one after a root canal, but it's not part of doing a root canal. I find it very odd that he didn't recommend you get a crown, but that's something you will have to ask him about. He may give you a discounted price on the crown now, since you didn't have it done back when you had dental insurance. It is worth at least asking him about to see if he will cut you a deal. My husband had a root canal, then 6 months later, when the insurance renewed at the end of the year, he had the crown done. You do have to be really careful with uncrowned root canals. Sorry you are in pain, tooth pain is some of the worst, and you need to have it taken care of because you can get a really nasty infection in there.

As far as suing him for malpractice, unless he told you specifically not to get a crown, as you wouldn't need it, then there really isn't much you can do as far as a lawsuit being successful against him. Although I would find a new dentist, as he should have discussed getting one and why you should get one. Doesn't sound like a great dentist IMO.