how a healthy old man he is! 哪里错了？

how 的感叹句结构是：how +adj. + a(an) +n.+主语+动词
所以应该是how healthy an old man he is.
或者把how 改为what。

应该是What a healthy old man he is!
what + 名词结构
how + 形容词结构

How healthy the old man is!

how healthy the old man he is或What a healthy man he is

how 改为 what