B12 deficiency is not necessarily the massive problem people think.
So much food aimed at vegans and vegetarians is fortified anyway. A single 8g of marmite gives you 80% of your daily requirements. So I get my intake every morning.
Vegan spreads and plant and nut milks are also fortified, as are a multitude of cereals and other foods.
Before I became vegan, I would never have bothered to check my iron, B12 or calcuim intake but now I am aware of it, I monitor our intake, regardless of whether I am pregnant or not.
B12 absorption also decreases with age so older people are advised to supplement too.
There is no reason to change your diet, but maybe take a bit of time to talk through things with DH and make it clear that his child will not be adversely affected. He is clearly supportive of your veganism normally so this is probably just anxiety. He will probably get comments from other people too so it will be good for him to be armed with some facts.
Everyone should have a look at their diet when they become pregnant. I wasn't vegan for my 3rd pregnancy and lost 2 teeth from calcium deficiency and was anemic throughout.