Hi Rina,
There are many multivitamin products that do not contain iron. I don't know if it's available in your country, but Centrum Silver is one well known multi without iron. Some of the chewable "gummi" multis also contain no iron. Minimizing sugar and processed white flour in his diet would be advisable. I would also suggest 100-200 mg vitamin C daily for its positive effect on the immune system. My own experience with candida seemed to be a result of a bad thyroid. If there is any history of thyroid disorders in the family, it might be wise to investigate. I don't know if there is any relation, but thyroid issues among thal minors are commonly reported. It may just be coincidental, since thyroid problems are common. I would also definitely request a vitamin D test, as low D affects everything and does cause a lowered immune system. Correcting D levels can improve a whole realm of things in the human body, so it's advisable to check this level. If you do, I would like to see the number because doctors vary greatly on what in needed, mostly depending on how up they are on the current studies regarding D.
Encouraging a nutritious diet is important at this age, and I do realize just how frustrating that is for parents, as my own 16 year old son is impossible and refuses to eat nutritious foods, but we have to try. Anytime I hear about possible problems with sugar, I get concerned that may be a road leading to diabetes later in life, so efforts do need to be made to ensure proper nutrition. There is much nutritional advice on this site, including an entire board dedicated to the topic. Reading some of this information may give you some other ideas on how to proceed.