How I make the winner realize he won?

Patience...

Just choose another winner if he doesn't claim it after a while?

Wait for him to mark the game as received. Contact support after a week if he activated the link but did not mark it as received.

Send him an email saying "You're Winner!"

you can always add someone from his friends list and ask if they can pass the message that he is being basicly a pain in the ass right now :P

Him blocking you after saying he won is a HUGE red flag.

If you are 100% sure he hasn't activated it yet, request a re-roll and make sure the new winner activates it immediately.

Ooooh, your in a sticky situation. Try to re-roll, you better hope that he dosen't trade or activate it later on.

It's a lesson, NEVER, gifts by e-mail, always add then send via text chat. Though if they have won a couple of gifts already and have decent CV, and im lazy. I just send via e-mail.

Key drop in forum? If he activated it, it wouldn't be a problem, if not, he won't be able to regift it.

Since you already sent the link, I would just wait the 7 days and then report him. Have your evidence ready that you sent it but that the guy did not activate it and has blocked you. You will get credit for it and the guy will get banned till he explains why he did what he did.

Suggestion for the future from someone who went though something just like this. Do not just send stuff though e-mail. Add them to steam friends. Send the key or HB link in steam chat. This way you know for sure they got it and ask them to activate it right away. Basically what I do is.

Add them to steam friends.
Once I see them online, I send a message to them. "Congrats on winning my steamgifts giveaway!"
I wait for their response. Once they do respond, I say "Give me a sec and I will send you the game"
I then send them the key, or HB link or game from inventory. I also hit sent on steamgifts.
I then say "There ya go, don't forget to hit received on steamgifts.com"
I check to see if they do it. Once they do, I send "Thank you very much, and enjoy your game."

It makes things much easier that way.

The first post had the correct answer. Patience. Your giveaway ended less than 8 hours before you made this post and the gifting process can take up to 7 days. If the winner doesn't respond by then, contact support.

Closing.