ESA3 ibr please helpppppppppp!

Fill the form in with details as they were in 2012 but use the 'Other Information' box (And a separate sheet if necessary) to explain what changes there have been and when.

There is no special form for this reassessment, it naturally asks for partners details, if that didn't apply during the relevant period then don't complete the partners details, again, probably worth mentioning that his partner moved in with him in 2014 in the other information box.

There's no reason for the form to be baffling, most of the questions are really straightforward if you take them in isolation, rather than just staring at the size of the form and thinking it's too much. Most questions are simple yes/no where you move on a section if the answer is no.

The letter is about potential underpayment. Until the ESA3 is returned it't not possible for DWP to calculate whether or not there is actually an underpayment. These forms should have been sent out when people were transferred to ESA but that didn't always happen which is why this exercise is being carried out now.
The form needs to be completed to show his situation on 15th August 2012. Any changes between then and when he stopped claiming benefit need to be provided either in the section at the end where there is scope to add more details, or on a separate piece of paper. If done on a separate sheet, staple it to the ESA3 so it can't get lost.
As he ceased claiming benefit in 2014 there is no need to provide information for beyond that date. If his partner joined him after he stopped claiming benefit there is no need to include anything about the partner. If they joined him whilst he was still claiming benefit, you will need to provide the date, and their employment and financial information from that date.