Ask him if anyone will kill him for being straight. Until he's under threat for his life and well-being for simply existing with certain biological characteristics, he's not being oppressed. He's being discriminated against.

There's a difference between being discrimination and true oppression. What your friend feels is discrimination in his exclusion from the LGBTQ+ community.

Discrimination makes people feel bad. Oppression kills people.

Your friend wants a space where he can commiserate about the discrimination he perceives as a straight cishet man. Your community wants a space where they can feel safe from oppression primarily instigated by straight cishet men.

Your friend's presence in that community, without additional context, is a threat. There are far too many stories of straight men gaybaiting the LGBTQ+ community to set them up to be robbed, assaulted, or murdered for anyone in the LGBTQ+ community to implicitly trust them.

The LGBTQ+ community isn't oppressing your friend, but they have good cause to be suspicious of and discriminate against him if they have no other information about him other than his demographic. He's expecting them to express patience for him whining about his life when people from his same demographic are actively trying to harm them.

If he wants sympathy from the LGBTQ+ community, then become an ally and actively work with them to protect those who are being oppressed. This is how you earn your place in a community and their sympathy.