I personally feel that his length is really good, when compared to his age. Now, whether or not he will attain 72 inches, that question may be hard to answer. There is so much variation, in regards to bloodlines, genes, etc.. Some lines may get 6 foot, some may not, no matter how much you feed him. While I personally feel that you should be satisfied enough with him reaching 5 feet and over ( I say that as a general statement ), you also may see him reach 6 foot as well.
My two biggest pits are around 6 foot, and my third biggest is right behind them. Regarding two of them, when I received them, they were adults, but they still grew at least a foot to a foot and a half longer in the following years. In other words, they were not done growing yet, still had growth potential, and continued to do so. They were not fed with increased feedings either, just appropriately sized prey on the schedule I normally fed my biggest adults.
Remember, your bull is only going to be 3 this year. I believe if he is 5 foot now, he has a chance of being 6 foot in the future. See how you didn't have to power feed, and he still grew? I personally would continue him on the same feeding schedule you have been doing, and go from there. If it ain't broke, don't fix it.
Post a pic of him if you can!!
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Genesis 1:1