(How) should I report this?

I've never done it myself, but I'm pretty sure you have to report him for not activating the games himself or something.

I guess:
Either go to http://www.steamgifts.com/support/tickets/new
Category: User Report
User: username
Or go to the user's profile page and click the report button (explamation mark) under their avatar. It'll autofill the category and user options.
Reason: Not Activating Won Gift
Description (optional): other details.

Yes, report him with Not activating won gift as the reason

http://www.steamgifts.com/about/faq

The winner has redeemed my gift, but not marked it received. How do I proceed?

• If your gift has been activated or redeemed by the winner, and they have not left feedback after seven days, please create a support ticket to request received feedback. A support member will review and ticket, and if the gift is activated on the winner's Steam account, approve the ticket and update the feedback to received.

The winner has not redeemed my gift, and I'm unable to get into contact with them. How do I proceed?

• If you've been unable to reach the winner of your giveaway using e-mail and Steam after seven days of your giveaway ending, and they have not yet activated or redeemed any keys or gifts you attempted to send, please contact support to request a new winner. When creating a ticket, we ask you to include data that suggests you made an adequate attempt to contact the winner, such as screenshots of e-mails, friend requests, or gifts pending on Steam. If the request is approved, a new winner will be generated by the site.

I think I spotted a fake giveaway, what should I do?

• We typically allow all giveaways to run their course without judgement, and allow the giveaway creator a fair chance to prove themselves and to come through with the gift in the end. An exception is when the giveaway creator enters a description that clearly states the giveaway is fake, or if the description illustrates a lack of understanding of how the giveaway system works. Please refrain from posting any comments accusing the giveaway creator of posting a fake giveaway, and only create a support ticket if the giveaway is fake beyond a reasonable doubt.

Do with it how you think you should do with it.
The answers have already been provided, prior to you making this thread.