Question

He who has suffered the most in the cause should speak.( Begin with: Let him...)​

Answer 1

should be allowed to speak, is the correct replacement of the bold part. In the sentence, both clauses are connected by the subject "he", i.e. auxiliary verb 'has' and modal 'should'. The doer/subject of both is 'he' only.

Hence, 'should be allowed to speak' is the right choice.

Hope that helps!

Answer 2

let him decide what he wants to do??