Hi, Namaste.
You are in a very sad position! The fact that you have researched and found this site, and are looking for help shows your commitment to this person, and your desire to help him. I think there are many things to consider. You say that he is very close to you. Does he consider you very close to him? (You don't need to answer me, I'm not prying, just asking you to consider this.) If he does consider you very close, then I would think his not wanting you there could stem from his own fear, and his uncertainty about what he will be like after surgery. Perhaps he wants to spare you having to see him after brain surgery, and is afraid that you would be burdened by having to help him afterwards. His fears will be exaggerated, I'm sure. The best thing that you can do is be frank and honest with him. Tell him that he means a lot to you, and that you want to be there to help him and support him in any way that you can. Tell him that you care and want to be with him to help get him through this. After that, it's up to him to accept the help. All that you can do is honestly let him know that you are there, that it is not something that you want to avoid, but that you are genuinely interested. He may still want to push people away - his way of dealing with this and coming to terms. That doesn't mean that he will always feel that way, though. He may initially push away, but then need people to come closer. He's scared and doesn't know what to expect. Perhaps one way that you can help him is by researching this - for him or with him, so that he can see what to expect, and can see that he can and will come through this and still live a reasonable life. That may help calm his fears and do more for him. How large is his AN? How old is he? Is surgery his only option, due to size of the tumor? There are a lot of questions that will help you find the answers for him. Perhaps having a calm person with him as he journeys through this, and someone to objectively look at the information and help him sort it out, will help put him at ease. Perhaps helping him make a list of everything he needs to do pre-surgery (cancel appointments, bills to be paid, house/apartment to be checked when he is away, someone to water the plants, bring him back home, etc.) might help him organize for his surgery?
In the end, all that you can do is extend your hand and make the gesture. If he does not want help, it doesn't mean that you aren't the person that he wants to help him. And it doesn't mean that his feelings about accepting help won't change in the future. Don't push him, just be there to reassure him. Don't judge him, and try not to get frustrated. Perhaps you can help get other friends of his to support him as well, and help with things that need to be done. Hey may be overwhelmed and not wanting to impose on people.
You've done a great thing by asking others how you can help him. There are many people here who have gone through this before. I haven't. They will be able to offer help knowing what's happening on 'his side' of the equation.
All the best. I hope this works out for you.
John