There's not a lot of detail of the history with you and your friend, so I'll try to post some general ideas that may or may not pertain to your situation.
How long or close friends are you? Is this behavior normal for him - to hold grudges? Or is this unusual? How long ago did he find out his diagnosis - is this recent?
If you are close friends, and this behavior is unusual for him, and his diagnosis is fairly recent (within 6 months to year) - there could be several explanations. Given the tumor is in/on his brain - that alone could account for unusual behaviors as who knows what that tumor is affecting. It's not unusual for people with brain cancers to have behavior changes and suddenly act different than normal, then a few months later all is better again. That's just one of the symptoms of the disease.
Another possibility is if it's a fairly new diagnosis, most likely your friend is scared and/or in shock - definitely is probably fighting anger as well. That's another reason he may act aggressively as you've described - where it's sourced more out of his own fear and anger, than any true feelings toward you. It's another possible reaction that is common. Sometimes this "lashing out" is a defense mechanism to push people away in an attempt "to make it easier for friends/family" if his prognosis is not good.
There could be several other explanations - including the behavior you describe is just his normal behavior (which if this is the case, well, as others say, you just can't change him).
Whatever the situation - if you are a true friend, pray for him/his family/friends - and let him dictate what he needs/wants from you. If you've made it known that you are there for him, the next move is up to him. Nothing you can do beyond that for the moment. If you think this behavior is a result of the disease, then don't think badly of him and don't take his reactions personal - just know it's not really "him" saying those things.
I wish you and him the best.