彼は太りすぎているのでダイエット中です。

He's on a diet because he is a little overweight.

彼(かれ)は太(ふと)りすぎているのでダイエット中(ちゅう)です。

Kare wa futorisugiteiru node daiettochū desu.

彼 (かれ)

he; him; his; boyfriend

は (は)

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

太る (ふとる)

to grow fat (stout, plump); to become fat; to gain weight

ので (ので、んで)

that being the case; because of ...; the reason is ....; given that...

です (です)

be; is

Readings: ヒ、かれ、かの、か.の

Meanings: he, that, the

Readings: タイ、タ、ふと.い、ふと.る

Meanings: plump, thick, big around

Readings: チュウ、なか、うち、あた.る

Meanings: in, inside, middle, mean, center