I can't say for sure, because there are so many different varieties of sorghum, BUT my experience has been that once they put the seed head on, the tend to add some height.
Again, your results may vary, but I would expect it to grow another foot or so...most sorghum strain reach between 4 and 5 feet.
Stay patient, there's plenty of growing season left, and we're into a decent dry spell. Some water and some lowered Kelvin rays (autumn sun) can really change things quickly.