Are you certain it's his iron that is low? There are many kids of anemia. IF it is iron, I'd wonder if he was not absorbing it properly, especially given the history of leaky gut.
Something you can look into are cell salts and he'd be a great candidate. They are the 12 inorganic materials necessary for life function and they assist in assimilation and digestion. I would look at bioplasma for him which is a combo of all 12. You can also look at ferrum phos (iron) on it's own. These are extremely helpful for kids and the elderly (those with the weakest digestion.)
Other than that make certain he's getting enough salt. Good salt. I would also say that herbal infusions are easy to make and can be used as the base for smoothies. Alfalfa, nettles and yellow dock may be a good place to start. Liquid chlorophyll is a great option as well.
Many doctors who work with kiddos with damaged guts do recommend meat ( and with good reason if you understand the biochemistry of what's happening ) but that doesn't mean it needs to happen. As long as you are diligent and watch the child...base your decisions on what's happening with them. Our bodies all function differently and it's important to remember that. Conversion can be an issue for them, as can processing. I'm not saying feed meat by a longshot. I'm just saying be open to what's going to be in your child's best interest at all times.
Foods that are high in iron are blackstrap molasses, apricots, dates, cherries, leafy greens etc. The dark circles under the eyes in my experience have quite a bit to do with mineral stores. That's why you often see them in "allergy kids." A leaky gut/candida/parasites/bacterial imbalances etc. all lead to difficulty in absorption and then, a deficiency in certain nutrients.
And just as a shout out....natural iron should not ever cause constipation. Synthetic iron almost always will.
I would also make sure the vitamin A and D as well as zinc stores are sufficient for your kiddo. Do you know the cause of the leaky gut?