First of all you should ask him where does his belief that evolution is just a theory stems from.
Its unlikely that its either of the first two. That just leaves #3.
The you can ask him how can he discredit 150+ years of scientific study, research & observation which has been studied & reviewed by scientists from all over the world based just on his personal opinion.
If you look at biological researches being conducted these days, every week there seems to be a new research that is the exact opposite of the old one. One week something is good for you, the next week its not. In this topsy-turvy world of biological research there seems to be no evidence found that is contradicting evolution, for over 150 years!
But lets assume that evolution isn't true. Then what is the alternative? Idiotic, err Intelligent Design? But then where did that intelligent designer come from? Did it just appear out of no where? Isn't it a much much much bigger leap of faith to believe in an intelligence that designed so many diverse things, some simple some complex, and also designed the rules on how they interact with each other, without any proof of its existence that it is to believe in 150 years of meticulous, credible, peer-reviewed scientific work which is supported by evidence?
His own research, as in scientific research or the research we do - read popular science books & internet articles?
Intelligence, and sound research do not always go hand in hand.
A lot of what Akshay said is where i would go with it. An important distinction that does not get made is the way "theory" is used in language. The word theory is not used the same way in science as normal people use it. We use it in conversation as a guess. A scientific theory is something based off of known facts to make testable predictions. The laryngeal nerve is a perfect example of evolution. Dawkins actually has a great video showing how in a giraffe it goes all the way down its neck only to come back up the other side rather than just going the shorter distance across the neck. To say that there is a mountain of evidence to support evolution is an understatement. I'd ask for any evidence to contradict 150 years of peer reviewed research as Akshay points out. He may try and poke holes, and ask rhetorical questions, but keep sticking to evidence and prepare for him to get frustrated and make non-sequiturs, straw man arguments, and ad hominem attacks.
I never said that.
All I said was that belief in god, for which there is no evidence requires much more faith than belief in evolution for which there is a lot evidence.
Ask him to define the word "theory" in context. There is the "scientific meaning" a collection of facts the theory explains, or you have the non-scientific meaning like, The moon is made of cheese, there are angels, there is a god, ect.
This "Just a Theory" argument is simply a word game with no substance.
The validity of theory is it's repeatable testability, explanatory factor (as measured by statistics) under changing conditions, and the ability to use it as a predictive tool. No theory is perfect, but there is no alternative theory to evolution which offer any empirical evidence, has any explanatory factor worth concidering, or any theory which can lead to accurate predictions.
Evolution takes no faith. The evidence for evolution is there. There's not one supporting any other theory. It's the same as "beliving" in the "theory of the existance of the rocks". As that debater seem to have one firmly planted on top of his shoulders, he might relate to the example. ;)