He never cries for no reason so when he does cry, he must have really hurt himself.
He very rarely cries so when he does cry, it must really hurt.
「たいしたことではいつも泣かないから、泣く時は、そうとう痛いんだね」= He never cries for no reason so when he does cry, he must have really hurt himself. / He very rarely cries so when he does cry, it must really hurt.
※ 女の子の場合heの代わりにsheを使います^^
ボキャブラリー
never cries for no reason = 理由なしで泣かない
when he does cry = 泣くときは (doesを使うと「泣く事」を強調する)
must have ~ = ~に違いない、~だろう
really hurt himself = そうとう痛い
very rarely cries = 滅多に泣かない