I have a brother in Christ that challenges every doctrine of belief. His heart is right and he is very dedicated to God and deep into ministries. But sometimes his questions cause division. For example, questioning the trinity, debating free will vs. election, questioning the divinity of Christ , question if works are a part of salvation, etc. These things divide us Christians instead of unite us and confuse new believers. They even cause me to question some things that I fear I ought not question. I know his heart is right, and he's not trying to 'serve his own appetites' but instead search for the truth. But sometimes I wonder if he's there because God wants me to question things deeper or if he's a stumbling block.
I think always turn to the Word of God, that will guide us. You will need to show him out of the Bible what God says about these questions. Knowledge of the Bible is key. In this way you will help him find the truth.
(I did not study Christianity at a university, but it will be wise to ask somebody that did study further on this subject, as they have a better understanding, to assist in get a better understanding). So in this way God is helping you through your friend to get a better understanding of His word. If you fear that he will influence you wrongly, it will be best to avoid him.