Sir,
Is there any difference between a question and a questiion tag?
Is there any difference between let and let's.
According to me, let's is an auxillary verb which is used for a polite request for which the confirmation question tag is ' Shall we?'. As in your reply you stated that let him go is an order. If we have to use the same sentence to seek the permission, then we have to use a question and not a question tag. If it is an order we do not have to use a question tag.