Well, not everyone grows at the same pace. I have seen some former classmates outgrow me in a few years around the age of 18. They were like half a head shorter than me before that. How tall are you, your husband and your son currently – if I may ask? You probably could also look at charts for children sizes if you find any.
IP6 seems to simply be phytic acid, which is usually not liked in food because it binds certain minerales so that the body can't use them anymore. I guess it is used in thalassemia to bind the iron? A side effect would probably be that it then also binds other important minerals. I'm not really sure what the sense behind IP6 would be if you already take an iron chelator. But maybe the forums have some information on this. Try to search for it.
Growth hormones: I don't know about the exact values, but if they found out he is deficient in growth hormones, I think they should really treat it directly, not via supplements but with injections. Growth hormones seem to be insanely expensive from what I heard so I'd recommend you to research certain values yourself since many doctors often try to save money if a condition is not 'life-threaning'. If unsure, try to get a second opinion from another doctor. If he keeps being deficient, I'd say try what you can to make them start growth hormone injections.