This will in all likelihood not be a problem.

The first name matches, middle names are rare on reservations anyway.

I have had typo's and mistakes with first and middle name and never had an issue. The last name and first few letters of the first name are what matters.

If you call KLM they will refer you to Expedia who might issue new tickets for the full cost of reissue!

Aside: can he edit the APD at klm.com? You can't edit first and last names but might be able to edit/correct/delete a middle name.