Hello and welcome to the forum, The issue of weight control can be tricky. You can find some information here
As you can see from that discussion it can take a lot of playing around with portion size to get the correct amounts. As he seems to do well on the Simpsons, it may be worth keeping him on it and reducing the portions even more. No one can really tell you if he has an allergy or what might be causing a reaction but as the problems started when you changed the food, it is likely that something in the new foods doesn't agree with him. If you enter 'allergies' in the search box, you should find lots of threads with some good advice about how to go about starting an elimination diet to help you identify the allergen. Alternatively you could have him tested.
What is the situation with extras? If you were looking at an elimination diet, you would need to ensure that at least at first, your dog only has your chosen food. No treats would help with the weight as well.