Or he might read the Microsoft Word Catalogue/Directory Mailmerge Tutorial in the 'Sticky' thread at the top of this forum:
https://www.msofficeforums.com/mail-...-tutorial.html
The tutorial covers everything from list creation to the insertion & calculation of values in multi-record tables in letters.

Since the tutorial document includes working field codes for all of its examples, most of the hard work has already been done - you should be able to do little more than copy/paste the relevant field codes into your own mailmerge main document, substitute/insert your own field names and adjust the formatting to get the results you desire. For some worked examples, see the attachments to the posts at:
mail merge with duplicate names but different dollar amounts
Access to Word, Creating a list from multiple records

Another option would be to use a DATABASE field in a normal ‘letter’ mailmerge main document and a macro to drive the process. An outline of this approach can be found at: Many to one email merge using tables - Microsoft Community
Conversely, if you're using a relational database or, Excel workbook with a separate table with just a single instance of each of the grouping criteria, a DATABASE field in a normal ‘letter’ mailmerge main document could be used without the need for a macro. An outline of this approach can be found at:
Mail Merge - To a Word Table on a Single Page - Microsoft Community
For some working examples, see:
Mail Merge: Using One Excel File with Multiple Sheets
Merge excel list into Word Receipt

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]