You mention that your dog has a sensitive stomach but have not said how this manifests itself. Does your dog refuse food, vomit, have diarrhoea or both? Also, have you been able to identify any trigger factors? Is there an ingredient that you know for sure he cannot tolerate? IIRC Canagan has chicken in it - is it possible that your dog has a chicken intolerance? This is not an uncommon one. It would maybe make sense to avoid chicken for now. You can always try a small amount at a later date when you have him settled on a suitable product.
Broadly speaking, with dogs like this it is sometimes useful to start with food consisting of a very simple ingredient list. Wet food is a good choice because they often meet this criteria. Fish is usually highly digestible and a good choice for dogs with digestive issues. If you know your dog tolerates fish and you choose to go down this route, there are a number of decent quality wet foods based on salmon or white fish and potato. Pets at Home sell their own wet foods based on fish - IIRC there is Wainwright's and Fishmongers. However, you could use the Dog Food Directory to source more if you wish.
Another wet food that is useful for dogs with digestive issues is Naturediet
and if you check out their website, there are some information sheets about this very issue. Again, it might be prudent, for the time being to avoid chicken.
Cold pressed food is a good choice. I am not fully au fait with Tribal so cannot comment at this time. In your case Gentle Fish
would be worth looking at or if you want grain free, Guru Full on Feast.
AFAIK neither contain chicken. The latter has duck and fish as the protein source. As Tinyplanets has said, brown rice is known to have good nutritional value and from my own experience I find it excellent for promoting good health in the bowel. If you decide to try cold pressed food, please be sure to weigh the food out accurately because it is very easy to overfeed due to the denseness of the product. Do not use cups, mugs or any other receptacle to measure the food. Overfeeding can cause diarrhoea. The RDA is 1% to 1.2% of the dog's ideal body weight. There is a good transition guide on the Guru website. They recommend that it can take up to two weeks to transition although some dogs might not require this length of time to adjust to the food. It can be softened with warm water (never boiling) to make the consistency of your choice.
It would be very helpful if you could update your thread from time to time so that we can follow progress. It might be useful for other people who are facing similar problems.