In this scene, Charlie figures out that Raymond was, in fact, the "Rain Man" from his youth that he had thought was imaginary.
They have a nice moment singing the Beatles' "I Saw Her Standing There." However, then Raymond freaks out when Charlie turns on the bath tap, saying that "hot water burn baby."
Apparently, Raymond was involved in an incident with hot water and a baby (i.e., Charlie), and that's why he went away—he had accidentally hurt Charlie.
Charlie helps him get ready for bed.
Parlez-vous a turning point in their relationship?