Try searing on all sides on a really hot grill and then finishing it in a 225 degree oven for five minutes. Then turn the heat off and leave it in the oven for 10 more minutes, and then let it rest out of the oven for 5-10 minutes, or longer if you can, before slicing.
You will get perfect rare all the way through with a warm red center.
It's the only way to get a porterhouse that thick done properly, if you try to grill it to finish it will over cook or under cook everytime.
The oven and resting steps allow the heat to distribute evenly through the steak.
I also put a couple of big pats of butter or some EVO on it before putting it in the oven to keep it happy.