I'm not too sure but I will try. An abstract painting is a painting that isn't very clear to people or that doesn't depict a clear image (It doesn't make much sense to anyone but the person who drew it). When people remember stuff sometimes it isn't very clear, or if they try to explain it to someone else the other person doesn't understand it well, So all in all, I believe the author is trying to say that a memory is usually unclear or foreign to anyone but the person who had that memory.
Answer:
Question is incomplete!
here you have to atleast mention that what and who are the speaker in this question and which chapter this question is referencing! :)
I think the line 'Since he was old enough to know, big boy' is the best line in the excerpt that uses metonymy. This is because the phrase 'he was old enough' is just an expanded meaning of the phrase 'big boy' - which is what metonymy is all about.