You need to find out if Contractor C is an employee of J's company. On one side, C is J's step son so that they are related. On the other hand, J denies that he was aware of C's work. Sounds like that J and C may not be associated in business. If this is a correct assumption, J is not liable for this damage. Actually J can sue C for misrepresenting J's company.
From your point of view, you don't get much back money wise by suing unlicensed or fake signature. However you can use this fact to show judge how you are deceived by the fake paperwork etc.
The important issue is that they did lousy job for what you paid him.You may sue him for getting money back or performing the work to your satisfactory. You may want to proceed as follows:
1) Locate contractor C. You must know where he lives or works before you get him served.
2) You need to talk to him first to see if you guys can settle peacefully.You may warn him that you will take a legal measure against him if he refuses to cooperate.
3) File a small claim with the court if he fails to cooperate.
Good luck.