There are no expunctions for convictions in Texas. It's on your record for life. You should contact some local attorneys who handle post convictions matter, writs of habeas corpus. A writ is the only way to attack this conviction at this point.
You cannot expunge any convictions. The case must have been dismissed or you must have been found not guilty to qualify for an expunction.
Other than your assertion that you believed you could expunge your record based on the lawyer's representation, you are not stating a basis to file a habeas application. If you were not guilty, but pled guilty because of your understanding regarding expunction, then you may have a basis for habeas relief. (The failure to find a weapon, without more, is not helpful. All it would take is your dad's testimony for a conviction regardless of whether or not a weapon was found, if a jury believed your dad beyond a reasonable doubt.)
As far as the confusion regarding whether your case was actually reduced to a misdemeanor, the problem lies in that the case was disposed of in a felony court, even though it was reduced. It appears to those who check backgrounds, on the surface, to be a felony. If they bothered to look further into the computer records, they would find that it was reduced. I suggest you get a copy of the judgment and sentence as well as the plea papers which will show that the case was reduced to a misdemeanor and just present those with your explanation.
There is no one you can sue. No one did anything wrong except perhaps the lawyer misadvising you - but I'll bet he will deny that is what he told you.