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!"
Don't be shocked when you see my son. He's gotten really chubby since COVID.
この場合、次のような言い方ができますよ。
ーDon't be shocked when you see my son. He's gotten really chubby since COVID.
「息子見てびっくりしないでね、コロナ以来すごく太っちゃって」
to be shocked「驚く・びっくりする」
to get really chubby で「すごく太る」
ご参考まで!