Question: I am an editor for a trade publication with 40 years of experience in journalism. The company wants me to take a job on the events side, writing content, helping with organization, moderating panels, archiving video. They haven’t called it a promotion but it would clearly be more intense work and involve traveling to conferences every other month. I’d like to accept the offer but I want to negotiate a salary increase after a six-month review. Can I do that upfront or do I wait the six months and in either case, what kind of percentage would be equitable?

Answer: 
Since you are using a versioning system: Fix one bug in his code and check the fix into the versioning system. Then fix another bug and check the fix into the versioning system. And so on. So when you need evidence you point your boss to the versioning system, which contains your name 99 times and his name once.

(In a professional environment with experienced developers you use a source code control system in a different way).

And consider that many companies use an internship as a very, very long interview. If that is the case in your company, then by the sounds of it you are passing the interview, and your colleague is not.
As other recommend, I would ask somebody higher up if I could start CCing him in all your exchanges besides documenting everything in a source code system. 

Obviously, if you working for the two, your colleague does not need to do it. However, it seems a bit odd he taking such a relaxed stance of missing so many hours of work and days off in the middle of an intern project...it is as he knows he cannot fail.

I would investigate if he has relatives inside. I have seen my share of pet employees over the years. It is usually not that hard to find out. Maybe you were setup together for a reason.

Welcome to the fantastic world of the real world and office politics!

Assuming no foul play, at the end of the day, someone is failing you not making regular meetings to guide you both and assess the state of the project.

PS. A possible course of action if that is a University endorsed internship is probing on the University side if you can change for another place. The odds are slim, but without asking it is not possible to find out.