Consensus by equine nutrition experts is that corn and corn products are not good for horses. Corn is incredibly difficult to digest and is one of the highest NSC's available.
For your horse, the Safe Choice Perform sounds like an excellent choice. I would even go a step further and top dress it with Cocosoya oil. Oil in the form of vegetable oil or Cocosoya oil (my preference) is one of the easiest, cheapest ways for a horse to gain weight.
In addition to the above, I strongly suggest adding a probiotic to his diet, especially due to his history of parasite damage. It will help him use his feed more effectively, I.e. Able to absorb more nutrients and also help with digestive stress, making him gain more weight. The probiotic I highly recommend and is the most bang for your buck is ProBalance bought at Horse.com. Good luck! For-A-Flex Pro Balance - Horse.com