Help needed with cooking a 2" thick Porterhouse steak on BBQ

Thank you for your great answers! I'm pulling it out of the fridge now to bring it to room temp. I was going to take it out one hour before cooking, but that advise of two hours makes better sense due to the size. I also unwrapped it yesterday when I brought it home and put it on a rack in the fridge to dry it out some. Since I get the filet, I hope this turns out great. Thanks again!

The thickness is perfect for a grilled medium rare outcome. I also start with a room temp steak and very hot grill. Oil the grate, don't worry if it feels stuck - it will release when seared. Give it a 90* turn for nice marks.
When done nicely turn it over. Do as the other side.
The touch test is correct - if you have done it successfully a few years. I say at this point use an instant read thermometer. And yes, move to a medium heat area off to one side.
Watch it like a hawk using your instant read.
I only use fresh ground pepper and fennel to start and a sprinkle of sea salt on the plate .
A pat of butter is fine if you like your streak to taste of butter. Restaurants use butter to cover "dry" areas of the meat when over seared.
As always start with a martini and then a good cab.

I was just coming back to add that you should take the steak out of the fridge for at least an hour before cooking..Pegreen beat me to it. :)

I copied this from somewhere. And in case you want to try it in a skillet, I had a link for a good Bobby Flay recipe.

How to cook a porterhouse steak on a gas grill

- Before you grill the porterhouse steaks, leave them at room temperature for 2 hours. This will keep them from over-charring on the grill.
- Season the steak with seasoning salt or salt and pepper.
- Make sure the grill is hot prior to cooking the steaks, at least 500–600 degrees.
- Resist the urge to flip them once the steaks are on the grill. You want a crust to form that will lock in the juices and flavor.
- Keep the lid closed while you grill the porterhouse steaks. This will allow the steaks to grill quickly and be heated and seared on all sides.
- Turn steaks over on opposite side in 5 minutes. Put lid back down. Let it cook for another 3-4 minutes to reach medium rare, or 5-6 minutes to reach medium.
- To tell the temperature, you can use a simple hand test to determine the doneness of your grilled porterhouse steaks.
- For medium rare, the steak should feel like the sponge area of your thumb.
- For medium, the steak should feel like the middle of your palm, a bit firmer.
- For medium well, the steak should feel like the base of your pinkie finger.
- A medium rare steak will have a warm, red center.
- A medium steak will have a hot, pink center

In a cast iron skillet:
http://www.bonappetit.com/recipe/perfect-porterhouse-steak

Yum, but oh my that is thick. I would do as you said and sear on both sides. Then lower your burners (on my weber Q, I would turn off the middle burner and turn the outer burners to mid low), shut the lid and use your grill as your oven. If he likes it rare, it will only take a couple of minutes. You may want to flip the steak halfway because the grills will stay hot even though you have lowered the heat.
A pat of butter after it's rested for ten minutes and there's a great birthday dinner. :0)