I recently dated a guy who was "vegan" (plant-based) in the past, who gave up and started doing the whole pasture egg, grass fed meat thing, but he still ate things like beans, nuts, and tofu curries. He said "I know I'm a hypocrite, I should be vegan, and I don't know if I can do it." I tried to convince him to be at least be vegetarian, like can you just eat eggs or something.
His overall personality appeared to be at war with itself, though. He claimed to want a relationship with me, then backed out suddenly and frantically (he ended it, not me, to be clear, abruptly and for no apparent reason other than fearing being trapped in a monogamous relationship) only to recently apologize, say he feels bad for the way he treated me, and blah blah.
I mean, I see a pattern to his behavior, of him wanting to do the "right thing" and yet a part of him self-destructing and making these decisions to eat animals, or run frantically from a relationship he claimed he wanted. He pursued me, won me over, then randomly let me go. I do see the same issue with his plant-based diet in the past, where he knows it's what he wants and yet a part of him is not willing to ...hmm...I don't know...exert the control required for things like being vegan or being in a monogamous relationship?
I'm not saying your partner's issues are the same, in fact they obviously are not. He seems to be committed to you, and also have no clear emotional problem with his own consumption of animals. But what I am saying is that you can probably look deeper into his overall psychology, just like I did with my guy, and see what the over-arching "issue" he has is. If you can find the heart of the "issue" then you'll know what the answer is in how to reach him in making him vegan, or at least vegetarian.
I think with the guy I was seeing, if we ended up getting back together, I would probably have to convince him that somehow our relationship was not "restricting" or "trapping" him in any way, and by the same token, I would probably have to show him just how many vegan products are out there, how fun veganism can be, to look at it as an adventure rather than a restriction, and probably cook him some super delicious food. His issue seems to be he fears his freedom being taken from him, or that his life won't be as "fun" in terms of sensual enjoyment. I can relate. When I went vegan I lived in LA and I loved my vegan products and trying new recipes and going to vegan restaurants.