In my answer to your earlier post about niacin, I mentioned that iron deficiencies are unusual in horses and since you horse's rbc count was only slightly low, it might be best to have his blood tested once again when he hasn't exercised strenuously the day before.
And, before giving an iron supplement, it would be best to determine the reason for the low red blood cell count. An ulcer is often the reason, especially in performance horses.
Most blood builders contain a large amount of iron, which can cause the production of free radicals. So, if you do end up giving an iron supplement, it should be under your vet's close supervision and have his blood tested within 3 months.
However, to answer your question, the Lixotinic that your vet suggested is safe for horses with HYPP. But, I hope you'll take into consideration my other suggestions.