" I want to forbid him to hang out with the boy but I haven't." This is probably a good thing. I would support you brother in his decision to see his friend, what ever his decision is. I have read stories of many gay who are rejected by their families committing suicide. Others who are rejected by their families just end their relationship with the family. Would you rather have a GAY brother or NO brother? Does it matter if your brother is GAY?
As for abuse not making your brother gay... There are many factors that affect sexuality, 1 of them is environmental including abuse, but this only 1 of many factors we studied in "Human Sexualty" at college... Science still dose not fully understand it, but early sexual experiences do affect sexuality, but don't control it by any means...Having an older brother has more effect on the likely hood that someone would become gay than being abused, this is a well known scientific statistic just so you know (my personal belief is when the older brother becomes the care \ emotional support giver this bonding produces the same effect as a boy who chooses a mate that resembles his mother, but this is my untested hypothesis)... Again I ask, does his sexualtiy matter? He will still be your brother, you should tell him you will love him either way, and try not to judge. Acceptance will strengthen your relationship... even through you said nothing, he may have been able to tell your reaction through body language...
I don't know how old your brother is, 2 years might matter, it might not... It depends on how old your brother is. If he is over 16 he might be ready for a relationship and this up to him, not you... You should protect him from being taken advantage of and perhaps ask him some questions to make sure, as abuse victims become vulnerable to more abuse... But remember you can only inform him, (unless he is really young, then you need to protect him) if you try to make these decisions for him (late in teens or later) he may resent you for it.
I don't know for sure as I do not know your mom... but based on my experiences, your mother is calling your brother a liar because it is easier to place the blame on your brother than to accept that she failed him... To accept that he really did get abused is to take on some of the guilt you have for not knowing it was happening, while you saw signs you had no knowledge and should not blame yourself, but victim blaming is quite common, especially when the alternative is self blame. The embarrassment is more on your mother than your family... My mom had to go through counseling after I disclosed my abuse.
“Your only limit within reason, is the one that you set up in your own mind.” Napoleon Hill, The Law of Success, 1925.