Don't be surprised when you meet him, because he's really gained a lot of weight.
"Don't be surprised"＝「驚かないでね」
"when you meet him"＝「彼に会ったとき」
"because he's really gained a lot of weight"＝「なぜかというと、彼の体重は相当増えたから」
Example in context:
"My son has really gained weight during the lock-down, so don't be surprised when you meet him next time!"
アメリカ合衆国 
