Hi violetbeth I'm not a person who has adopted so I am not aware of the extra difficulties that may go with food issues etc but I have had a share share of food issues and I've also has my share of issues with me and DH not always agreeing how to parent our one DD!
I'd agree with all the wise advice here and think you and DH getting on the same page about parenting and food would be very helpful/essential.
How kids eat at a certain time does not mean it will always be the same.
DS will expand his diet and he is actually eating quite well. But putting pressure on him will probably only lead to things going wrong in other ways. Much better for you all to adapt to each other and get along as a family and gradually he will probably feel more adventurous with food.
I would also recommend a play element with some foods. If you want to distinguish it from main meals maybe make it a fun time mid morning/ mid afternoon etc for a snack.
I often did stuff with DD like fruity faces - make faces from fresh and dried fruit and then eating it - some tinned fruit is also nice and soft to cut (for you) and easy to chew for him.
We have also made snowmen out of ice cream and decorated them with raisons and cornflakes (instead of coal and carrots!).
I usually take a photo of the fruit face or snow man so it is also an opportunity to praise the artistic skills as well. You can make one (and eat it) he can follow your lead - if he wants to!
It's a chance to get messy without worrying about it (new to some kids) and also if the food does not get eaten then don't stress about it being 'wasted'. An opportunity for fun and play is not a waste (as kids we made endless pictures from dried pasta and lentils which no one felt compelled to eat).
All the very best with your lovely new DS, I am feeling misty-eyed remembering the fun we had with food and I am sure you will have lots of fun to come.