The company must have an appeal process or you need to follow the ACAS procedure for appeals.
He is appealing on the basis that the procedure was not followed.
The fact that a procedure was not followed does not in itself invalidate the decision.
The issue is whether any prejudice has resulted and whether it would have made a difference to the decision.
He has not denied the substance of the allegations
On the first point there is a difference in the phrases used as a "formal" oral warning is not the same as an informal warning
You can simply acknowledge this and confirm that his employment record has been amended accordingly.
He did not get 3 clear days notice.
You can acknowledge this but it is difficult to see how this might have prejudiced him , he raised no defence and had nothing to prepare or consider . He has not indicated how this caused him a problem.
The bot***** *****ne is that he is correct but the deficiencies have not caused him any prejudice as he does not deny the substantive allegation and any procedural mistakes did not affect the outcome. Confirm the result and make sure the 6 months runs from the original decision , not this one
I hope this helps.If there are any further points please reply I will be happy to respond