As the previous post explained, Boo Radley's brother did not put cement into the tree. Boo Radley's father, Nathan Radley, filled the hole in the tree with cement. In chapter seven, Jem asks Nathan the reason for filling the tree knot with cement.
“Mr. Radley, ah—did you put cement in that hole in that tree down yonder?”
“Yes,” he said. “I filled it up.”
“Why’d you do it, sir?”
“Tree’s dying. You plug ‘em with cement when they’re sick. You ought to know that, Jem.”
Jem asks Atticus about the tree's health, and Atticus believes that the tree is quite healthy. Jem, Scout, and readers are left to believe that Nathan Radley is lying, and that he put cement in the tree to stop his son from leaving gifts for Scout and Jem.
Readers are never told how Boo feels about the knothole being plugged up with cement by his father. Part of the reason is that the story is being narrated by Scout. We know her thoughts throughout the story, but we only know the thoughts of other characters if they speak about them. Boo has very minimal spoken interaction with Scout, and the tree is not a topic that Boo and Scout talk about near the end of the story.
Readers can probably be quite sure that Boo Radley isn't happy about having cement put into the knot of the tree. That knot allowed him to "communicate" with Scout and Jem. Nathan Radley took that away from Boo. It's possible that Boo was angry about his father's actions; however, it's equally likely that Boo was sad or frustrated instead.