Hi there!
I don't think there's anything wrong with feeding the same thing several times a week. As you note, one week they'll love something and the next week they'll refuse it point blank!
DD is almost 13 mo but we give her mostly the same meals (sometimes even the same thing for lunch and dinner!). She loves pasta with meat sauce, grilled cheese, veggies cooked in chicken broth (potatoes, celery, carrots), and we have mexican alot so we modify it for her by taking the ground beef and onion before we add the taco seasoning, then mix it with brown rice and sauteed peppers and onions and a little bit of salsa for flavor and chop it up in the food processor. It's messy (no spoon yet for her) but she she eats it by the fistful.
These are pretty much the only meals she has. Breakfast is the only meal that really has any variety - yogurt, fruit, oatmeal, toast, eggs. I just try to look at each meal in terms of food groups and as long as she is getting fruits, whole grains, veggies, protein, and dairy, I don't worry.
If DS is gaining weight then I wouldn't worry about it all. All my friends with older toddlers complain about their picky eaters but maintain that as frustrating as it is, they know their kids are fine. And like you said, it will all change next week, right!

Maria