Oh Arimeh! Biggest from me. You sound like you are trying everything, and doing your absolute very best, so please don't think this is a reflection on you, because its not.

I would take him to a GP or paed. Maybe even trying acupuncture (my paed does it with what looks like a laser pointer!) or chiro. A nutritionist may be a good idea too, maybe they could give you a couple of different ideas to try as well. Just work through the list until you find something that works, but then again, DS may just 'decide' to eat something.

I don't think there is anything wrong with giving the toddler formula, the nutritional goodies in it will outweigh the 'downsides' of formula (for DS, I doubt there would be any atm). Bug has rather a lot of it, and I think she is going just fine.

Normal weetbix are very high in iron. I remember reading the nutritional panel and comparing them to 'iron enriched' farex, and child 'iron enriched' weet bix too, normal weetbix has the most iron. So if he'll eat weetbix, even if just a little bit, that is a great start. For something different, maybe try giving him the weetbix with juice instead of milk? Don't laugh, a girl I used to know couldn't have milk, so she would have juice on her cereals. At least that would help with the vitamin c too.

Another food you could try is the cold meats, oops, its already been suggested. Was going to say things like strass, or the smilie meat (hey, its a novelty thing!). May not be the best thing for him to eat, but it may be a start. Shaved chicken breast or shaved turkey breast is rather yummy.

Good luck, I hope you sort it soon.

Oh yeah. If he will eat a bit of icecream, you could try sprinkling some of the formula on it, if he won't drink it. Or try the smoothie option, or mix in a bit of yoghurt.