简单点的说法：
①He’s no longer popular.
高级一点：
②He's no longer in his prime.或He's now past his prime.（已过鼎盛期）
③He had his moments.（曾有过辉煌）
④He's a has-been.（过气人物）
 
可以表达“过气”的词组：
1 a has-been
a has-been指过气的人物。
If you describe someone as a has-been, you are indicating in an unkind way that they were important or respected in the past, but they are not now.
例句：
He became famous overnight / at that time, but now he is just a has-been.
他那时候一夜成名，但现在已经过气了。
 
2 over the hill
over the hill指风光不再，开始走下坡路。
从字面意思看，这个短语就是翻过山坡的意思，翻过山坡不就是要走下坡路了嘛。用来形容人的时候，就是指已经过了鼎盛时期了，开始慢慢往下走了。
例句：
I don't think of myself as being over the hill yet.
我不认为我已经在走下坡路了。
 
3 a spent force
a spent force指过气、昨日黄花。
释义：Someone who no longer has the power or influence he once had.
例句：
The new album is proof that this band is not a spent force just yet.
新专辑证明这支乐队还远没有过气。
 
4 over party
over party指某人或某风潮过气了，在俚语中可理解为人走茶凉。
释义：When a trend or a person is over.
例句：
He's over party.
他过气了。
She should go to her own over party.
她过气了。