The 1st one says that his mother will scold him if he get home late, so it's a sentence in the present and that talks about the future. it didn't happen yet, but he is saying that it will possibly happen if he get home late.
The 2nd one is a sentence in the past, and it says that at the time he got home late, his mom scolded him. for example:
"if I got late at home, my mom scolded me", so I'm going now.
"when I got late at home, my mom scolded me." so I went upstairs to my bedroom.
Hope I have helped.