这句用英文如何说呢

比较随便的说法：Remember to say hello to him.
比较正式的说法：Remember to express best wishes to him.
若是代我向他问好，him 后面加个for me就OK 了

Don't forget to say hello to him

1.Please give my best wishes to him.
2.Please remember me to him.

Do not forget to say hello to him。

Do not forget to ask nice

Don't forget to say hello to him.