Wow sounds like you've tried lots of options! 
Speaking from a vegetarian standpoint, (and not wanting to sound bias cos that's not where I'm coming from!) do you think it could be the texture thing? I've read quite a bit on how wee children and kids often don't like the taste of meat and it's our persistence (or insistence) with it that makes them adapt? If it's important to you that he eats red meat then maybe keep trying the different textures but if you guys don't eat alot of it yourselves then in your position I'd be tempted to just keep inlcluding him in family meals that have chicken and or fish as a base and hopefully he'll copy you eventually.
From a health point of view, I'm not sure about those vitamin drops- it depends on the brand but alot of extra crap in them so be a bit careful about the quality. You don't want to be gicing him aspartame or suuchrin(sp) laced pills because he's not eating meat yet! 
The other thing to try if you're worried about his iron/protein intake etc is to maybe try stuff like hummus on toast or blend cooked red lentils through tomato based sauces... We do this with our non vegetarian step kids and they never know lol. 