I didn't read all the other posts, so I hope I'm not repeating anything.

What about Tofu? You can buy some that tastes like meat & other things. Its full of veggie proteins & other stuff (or atleast I think it is).

Also there are veggie burgers (but make you own as their might be onion in the premade ones)! Those taste pretty good & to a dog I'm sure it'll be a great taste to him!