This is just a guess, but I see that you also have a new baby, another boy. It is possibly that your son was used to being your "baby" and the only boy, and at seven it is very hard for him to accept that that has changed. A 21 mo old is pretty demanding of parents' time (and he is the child of your new husband, right?) and the baby is likely to get the attention of outsiders as well. I think your sense that he feels abandoned may be accurate - but I would guess it is at least as much because of your new family as it is because of your daughter.
If I were to make a suggestion, it would be to try to catch him doing things right. When I wanted to do this with my daughter, I woke her with a bright happy voice and said "look at me!" in an excited tone. Of course she looked, so I popped an M&M in her mouth and complimented her on doing what I asked so quickly and nicely. She caught on in no time, and we had a lovely morning and great day, and a real turn around in her behavior. You could also set up a reward system (this always works better with my daughter than a punishment or consequence) and agree that good behavior at the store, or helping with chores, or whatever, will earn a game with you or a trip to the park. You can also name it for him: "Sweetie, I've noticed that you've been having a hard time lately, and I think you may be feeling left out. It can be really hard when a family changes. But we are a family and Mom and Dad will work with you to help you learn to handle it. We love you very much and you are very important to us." (This can be when you offer your reward incentive.)
And of course, kids go through stages for no apparent reason. And just when you think you have things figures out, it will be something new....
Good luck!
A.