Tough one, my DD isn't big on meat, but sounds like she is better than your poor boy at the moment.
I've been really Iron deficient and it makes you feel like crap, if you pull his lower eye lid down and it's white and not pink then he is probably deficient. ( I think that's how it works, my brain has gone out the window lately)
I was gonna suggest maccas cheeseburger for a while that was all the meat DD was eating.
Not sure what else to suggest, I actually got DD to eat Pate for a while, I know who would have thought, just on crackers, I called called it special paste. Maybe you could call it dog food or something, he might eat it if he gets on well with one of the dogs.

Def go the vitamins, there is a liquid iron suplement that you can get, a trip to the gp might be an idea and take it from there.

ETA maybe you could make smootie with the toddler formula, what ever his favourite flavour is.