A few ideas to try that I've done with my fussy eater.
One is just to serve a family meal and if he doesn't eat it, he goes hungry till tomorrow. Sounds mean but if he's using it as control, there's no battle to be had. Can't do this if he's underweight, but if he isn't - he's OK.
The other is put out some easy stuff - chopped cheese, bread, fruit, crudites etc and let him pick something from each food group without commenting on what or how much. My son was soooo fussy for ages until I worked out that the few things he did eat were incredibly healthy - salmon, humous and cheese, pasta and rice, a few fruits and veg - enough to have some variety. Over time I just started putting food on the table and he leaves what he doesn't like.
Ask him to plan a menu - we played at restaurants and put candles on the table and he wrote out the menu with drinks choices etc and got to dress up if he wanted to. It's not as much a fuss of this as it sounds and if he's helped choose or cook the meal, he might eat it. You could let him pick a meal at least once a week, and even if it's always tinned spaghetti, make out it's a delicious choice and all eat it happily together so he associates food with happiness.
rewards/charts? Better to have a chocolate at the end of a meal than no meal and no choc. We used mild bribery a lot and it did take the battles away.
I know the best thing is not to make a fuss. Easier said than done - I never managed it!
Good luck with being a step mum.