First of all there is no reason to start acting any differently around your friend because he is still the same exact person, you just now have more information about him. You should feel honored that he chose to confide in you and this may change your friendship in a very positive way. It seems like you are having the right reaction. And it is okay to feel like something has shifted.

You should be aware of whether or not the situation is difficult for him and you should try to support him as much as you can. You should be open to your friend and your new perception of him. Just because your friend is gay there is no reason for him to be tested for AIDS. Do you know whether or not your friend has participated in any risky behavior such as unsafe sex or IV drug use? These are the reasons anyone should think your friend should be tested for AIDS.