Warning Spoilers!
What is Touji's link with Unit-03?
Theory
This is just speculation. I really don't think that Touji has any real link. Touji is the Fourth Child(ren). Meaning he is the Fourth level child as stated by Ritsuko. Fourth level children are all the children in Shinji's class, so techinically everyone in Shinji's class are classified as Fourth level children. Touji is just the most eligible candidate.
