Lack of appetite is one of the signs of iron deficiency. I would be seeing your GP for an initial assessment (and to rule out any other health issues) and then I'd be asking for a referral to a good pediatric dietician. It may be that your son may never chose to eat meat in which case you need to learn how to tweak the diet appropriately. The other thing to check out his general tolerance of tastes and textures, apparently some children have sensory overload when it comes to certain sorts of mouth textures, and it often feeds into speech issues as well (sorry I can't recall the name of this particular issue but it's something you can ask about and if necc get him assessed with a speechie if you've exhausted all options and it's still a problem).

Good luck, I hope it's sorted out with something simple like a particular supplement and . It sounds like you're doing a wonderful job caring for him!