I wish I wasn't the harbourer of bad news here and I don't want to come off pretentious or condescending but I suffered through it before I made the change: Pedigree contains corns and grains and animal by-products and basically is a garbage quality food.
I used to fed Purina (though I didn't have problems) and when I made the switch to Holistic food the change was incredible.
Now before it's said, no holisitc is not too expensive when you consider it. I am a poor student (and believe you me..Poor ain't even the word) but I was able to provide for my pup. I started late to Holisitc so I only tried a few.
Coco's favorite was Holistic Blend (18.99 for a 2kg bag if I'm not mistaken). HB cost me less than 20$ and lasted a bit over a month on 2 big meals a day. Coco was about 20lbs too so the size of a JRT. Best part is, Holistic Blend is available at some grocery store Natural Value sections.. Mine is at Loblaws.
Then there is Orijen. I had a sample and Coco actually ate that one dry!! (bc of age I soaked all his kibbles but he liked the taste dry on Orijen)
Chicken Soup for the Dog Lover's Soul was another holistic I tried. (we actually won the bag at the pet expo so I don't know cost). Some people like it and I hear it's a good food for the cost but I didn't like the results with my dog.
Finally, while it wasn't labeled Holisitic, Coco went out eating Presiden't Choice Nutrition first for senior dogs. It was really cheap but the ingredients were not far off from Holistic Blend. And it contained no corn/grains/by products etc. .
Most pet stores will carry the higher up brands. Most of them are filler foods (contains corn etc) but if you go to
www.dogfoodanalysis.com you can rate them. I personally find the sight a bit testy and objective. Because every dog is different you can call the shots.
For wet food, because I always mixed something with dry kibble, I fed "Lick your Chops". It was human grade, slow stewed holistic food. Made with brown rice and turkey and all the good things dogs love. I would be shocked if your dog didn't like it! Coco recognized the can before it was even opened and knew what was coming! There is weak debate about wet food vs. dry food. But I feel it is totally necessary to feed wet and dry in combination so the dog gets all vitamins and juices (oh, the succulent juices) to digest properly. Plus, dry kibble cracks off in the gumline and can harbour bacteria and possibly lead to gyngivitis. (oh dear, I ruined the spelling) so teeth brushing is more crucial to dogs fed dry only.
I would pick up some holistic food and start to gradually change it and see what results you get with Martin. While JRT's are considered a very healthy breed generally (bc of little human playing with the genes) they are still susceptible to irritation and problems from bad food.