I don't know if anyone else has suggested this yet, but if they have, kudos to them.
Write an essay (using as many scientific facts at possible) and ask him to read over it for you (give it to him at the end of the lecture) or somehow slip it into his paperwork. I would suggest you read "the china study" and get a bit out of that. Also, check out peter singer, he's pretty much written your argument for you.
If it were me I would have yelled across everybody and told him to shut his poorly educated mouth and go back to the pre historic cave he came from, as by cutting out the meat, we would have less greenhouse gasses, more land (think about it, the cows eat the plants, the people eat the cows.... so what if the people just ate the plants, resulting in less cows being mass produced, resulting in less space required.
I would also argue that it's quite common for a vegan student to have a better/more nutritional, yet still reasonably priced diet than an omnivore one, as a can of lentils costs a heck of a lot less than beef, and a LOT of transfats and cholestoral is instantly cut out of ones diet, as well as takeaway, causing the student to cook for themselves, hence think about what they are eating. That's not to say however, that there aren't vegan students out there who basically live of 2 minute noodles, but there are many omnivores who also do so.
I've had a couple of nasty incidents with lecturers, usually it's because of something they would show, one had a clip of a father and son going rabbit hunting which I can only assume was very graphic ( I walked out of that class and had a stern chat with them at the end of it) and the other played a clip where a dog was intentionally bitten by a snake and the class watched as the dog wailed in pain and eventually died... That lecturer also heard many harsh words and will not be showing the clip any time soon.
yes it would of been good for someone else to notice what he was saying without blindly talking notes 
Meet someone who should be dead according to his info. lol. Seriously is he using texbooks from 1950 or something? Vegetarian diets are preferential according to many organizations. Give him the China Study to read.
