Inactive recipient.

You should add him and ask if he's received the key and if it worked, mails which contain words like "winner" or "gift" tend to get flagged as spam so he might have overlooked the mail, if he doesn't respond simply contact support and ask them to mark is as received with proof that you've mailed the winner.
-edit- you could also request a reroll like RedCoats suggested, but only if it's a gift since you can't be sure that he didn't activate it if it was a CD key.

The submitter and winner(s) of the giveaway have 7 days to complete the gifting process. This includes the activation of the gift on the winner's account.

Make sure you try contacting the winner through Steam and email!

If the winner still hasn't activated the gift and marked the giveaway as received after 7 days, you may submit a request for a new winner. You will need to provide proof that you attempted to contact or send the gift to the winner. Please note that we do not normally approve re-rolls if a key was sent at anytime.

i've added him now on Steam to ask it, and i'll give him now some time to accept the request. TY for your advise anyways