Investigating low iron levels

[Dannygirl]

My DS aged 6.5 was diagnosed via blood test with low iron levels and has been taking a supplement (Sytron) for 2 years+. The supplement brought his iron levels up to normal but recently I became convinced the Sytron was giving him tummy ache (which I believe is quite common with iron supplements) so I stopped the supplement, and after a few weeks his levels have dropped very low again. He has a varied diet, eats red meat, eggs, green veg etc. and doesn't drink milk at all. My concern is that there must be an underlying reason for his low iron levels, and I don't know where to go or how to investigate this. The GP seems uninterested in finding out the cause and just wants him to continue to take the supplements despite the tummy aches. If you have any experience or advice in this area it would be much appreciated! Thanks