It 'may' be that insufficient insulin and/or insufficient food are causing him to lose weight. (This assumes that he doesn't also have kidney disease or hyperthyroid, which can also cause weight loss).
The Sainsbury's 'delicious' might be too low in calories for him if that is all he's eating.
There are lots of low carb foods in the UK. And hopefully there will be a food - or combination of foods - that suit Rasmus.
Incidentally, my old diabetic boy is 20 now, and it can be a struggle to keep weight on him. I get food into him at any possible opportunity! But I've found that including a high calorie (and higher fat) food in his diet helps a lot. At the moment I'm giving him some 'Little Big Paw' a couple of times a day in addition to any other food that he eats. It's a dense pate, but easily chopped up into pick-up-able pieces, and seems very palatable. He also gets a liquid cat food every few days called 'Liquivite', which I leave in a bowl next to his water fountain. It adds to his food intake, and also helps to keep him hydrated.
Are you weighing Rasmus at home? If not then I highly recommend getting a 'baby scale' so that you can keep a close eye on his weight. I got one recently for Bertie, and wished I'd done it ages ago. This one looks identical to the one I got recently for Bertie, and it works really well.
https://www.amazon.co.uk/OUkANING-Electronic-Digital-Backlit-Weighing/dp/B079Z64854/ref=sr_1_7?s=kitchen&ie=UTF8&qid=1553088424&sr=1-7&keywords=baby scale
Remember too, Kris, that it is still very 'early days' in Rasmus's diabetes journey. It can take a little while to see results. Do be patient. And be kind to yourself. You're doing a good job. Rasmus is lucky to have you in his corner.


Eliz