Hi Topaz and welcome,
The first thing you should know that there are three classifications of thalassemia. Thal major, thal intermedia and thal minor. You have not told us enough yet to know if your son is minor or intermedia but you can be assured that he is not thal major, as by age 4 he would be requiring blood transfusions to survive if he was a major. Did the doctor tell you what his hemoglobin or hematocrit level is? That would give us a much better understanding of his condition and what to expect in the long term.
You are correct that he should avoid iron and red meat has the highest concentration of readily absorbed iron, so that should also be minimized in his diet, if iron is a problem. However, iron studies, especially the serum ferritin test are needed to determine if his iron levels are high. If he is an intermedia, iron absorption from food can become a problem, so it is very important to find out what his classification is. The only vitamin that might need to be limited is vitamin C and only if iron absorption from food is a problem. In fact, it can be of much benefit to thalassemics, minor through major, to take vitamin and mineral supplements.
Please be aware that if he is a minor, one of his parents is also a minor. If he is an intermedia, it means both parents are minors and both carry the thalassemia gene. This is very important to know in terms of your future children and also information your son will need when he is older, should he choose to start a family. Awareness is the most important thing in terms of reducing the amount of thalassemia major births. All carriers should be made aware of their carrier status.
Please try to get more information about his hemoglobin level. It will be of great help to us so we can tell you more what to expect as he gets older.
The only cure for thalassemia is bone marrow transplant and is only used for thal majors.