I'm hesitant to recommend any one particular food because there are so many. I share your puzzlement about this because the product has 4.4 on fibre and already contains pumpkin, butternut squash and other things that should promote a healthy digestive system and colon. Are you sure that your dog is constipated? The true constipated stool is one that is hard and passed in small pieces - very much like rabbit droppings. I assume he has been wormed since you brought him home? It's two years since my youngest was a baby but IIRC worming should be done at regular intervals until they are 6 months. Of course the regime depends upon the product used.
Anyway, back to food. If you don't have any success with your extra measures, check out the
Dog Food Directory. I've just applied the following Filters:
Type of Food - dry complete
Food Properties - natural, hypoallergenic, clearly labelled.
Rating - moving the slider, I selected 4 to 5 because your current food is in this category.
Avoid Ingredients - all red ingredients.
You can tweak the filters yourself if there is something you don't (or do) want. If you want to increase the choice you can reduce the star rating. These are a good indication of quality but some dogs do better on products that are a little lower in them.
It returned four pages for me and it is then a case of going through each one to see which might fit the bill. Maybe it would be useful to look for products with brown rice and oats but also a good fibre value. For instance, Barking Heads Puppy Days has brown rice, potato and oats. Fibre is average at 3.3. Beet pulp is not thought much of by some but it does provide dietary fibre so it might be worth looking at products that contain this. Information about beet pulp can be found
here.
Hopefully your new regime of feeding might help your Dachshund's problem but if not there is plenty there to look at.