Dog with sensitive stomach - food help!

I have a 4 years old jack russell who has a sensitive stomach. The vet said to absolutely not give raw food and to not attempt to make your own food. The vet told us to give him low fat food and recommended Royal Canine Gastrointestinal Low Fat, but the clinic is also sponsored by Royal Canine so I'm not sure. I noticed he got better when I gave him grain free kibble, but we still have some minor issues. I have heard it's very common for dogs to not do well on chicken, so I've been thinking of giving him a grain free, lamb based kibble. I'm just a bit sceptical to Royal Canine as they aren't grain free.

I've been thinking about trying either Purenatural Grain Free Lamb or Eukanuba Lamb and Rice. It's just that my dog is very picky and refuses to eat if the bites are too big, and it's really hard to find information about the size of the kibble. I don't live close to any stores that sell good quality dog food so I can't go and check it out. Anyway, do you have any advice on what to go with? Would you go with the kibble the vet recommended, even though they are sponsored by them and the kibble contain grains, or do you have any experience with the other two, or maybe have your own suggestion? It needs to be something that is available internationally because I'm not in the USA.