don"t let him do that again这个句型对吗?

let 后用动词原型 所以是do

应该是do 因为let 后面一般加动词原形

don"t let him do that again
let sb do sth 让某人做某事，是固定搭配
雷同make/have do sth

let sb. do是固定用法，意思为“让某人做某事”。sb.后面用动词原型即可。

Don't let him do that again.是对的，也可以说成：Let him not do that again.

如果是说以后不再让他那么干了
应该用将来时, I won't let him do that again.