Nope.
Am I required to inform my opponent that I have him in "Check?"

Standard practise and etiquette say yes, the rules say no.
The rules allow saying check. However, most players beyond the scholastic level do not announce check. In fact, most players of average or above average strength will view check announcements as either irritating or insulting behavior. ("What, you think I can't see that it's check?)

quick answer to question , no
longer it is up to the opponent to know they are in check , online its less of a problem ie comp wont allow a move that doesnt deal with check ,so thats a giveaway , but otb , no , if opponent plays a move that isnt anything to do with the check , british chess fed states clocks stop person who checked gains 2 mins position is returned to the check and a legal move ie get out of check has to be played , the 2 mins esp in tight time trouble can be decisive

...most players of average or above average strength will view check announcements as either irritating or insulting behavior. ("What, you think I can't see that it's check?)
Absolutely. I heard of a fight nearly taking place at a recent tournament because the "checker" kept announcing it...

Standard practise and etiquette say yes, the rules say no.
Have you a link to such sources ?
As for me, I've never heard anyone saying "check" in real life.

Announcing 'check' online is very un-necessary.
You could PM your opponent...
"Pardon me, old chap, but I'm afraid that I have you in check."

Am I required to inform my opponent that I have him in "Check?"
LOL !!! No !!! This forum has a lot of very very very very funny questions.
If you want to see very very very very interesting games and analysis, go to:

in OTB check is never called unless your opponent trys to make an illegal move, than you wisper to him "you're in check"

Standard practise and etiquette say yes, the rules say no.
Nope. Just don't. Saying check usually shows that you don't know what you are doing. Of course you will see once in a while a guy say check check check. Just don't. No benefit for you, and it's not even moral. If he makes an illegal move, you gain 2 minutes.

Carlsen informs when he checks or mates during his blindfolded simuls. That's all I know. I thought you weren't allowed to speak to anyone but the TD.

Carlsen informs when he checks or mates during his blindfolded simuls. That's all I know. I thought you weren't allowed to speak to anyone but the TD.
No, you're allowed a limited vocabulary; Offering draws, resigning, saying check (if you want to risk antagonizing your opponent and the surrounding players).

Carlsen informs when he checks or mates during his blindfolded simuls. That's all I know. I thought you weren't allowed to speak to anyone but the TD.
He probably did that just to confirm things
Am I required to inform my opponent that I have him in "Check?"