Hi
I do hope the exclusion diet is helpful for you. Are you giving cooked or raw chicken? Presumably the potato is cooked but is the other veg raw or cooked?
Do check this, but I seem to remember a calculation for raw feeding puppies is that you should aim for 2.5% - 3% of their expected adult weight as a guide. I've also seen another calculation which suggests you feed them 8-10% of their current puppy weight. However, I'm not quite sure how this translates when the food is cooked.
As someone else mentioned you do need some calcium, and a good balance of meat and offal but 6 weeks of any diet is unlikely to do long term harm. Might be worth checking with your vet to see if chicken liver is OK though. This is only £1 in Tesco and I'd suggest little and often. You can cook it but I feed it raw - I blitz it in the food processor and freeze in small amounts (an ice cube tray or mince pie type tray is good). I add a small amount every couple of days or so. Don't forget that you can buy raw minced chicken in larger pet shops and you can cook it if you really want to - this will contain some ground bone too.
I've never had a Leo so it makes sense to get your definitive information from others more experienced in the breed but it sounds like you might not be feeding enough protein. I'd really be expecting the weight of chicken to be higher, perhaps 2/3rds meat to 1/3rd potato? I'd also expect that your boy would possibly still be on 3 meals a day?
As a for instance, my 28.5kg adult gundog is on 900g raw meat per day - just over 3% of his body weight, although he's a very active 2.5 year old. He's an itchy boy too so you have my sympathy. One problem with steroids is that it can make them pee more and very thirsty. A raw diet is great to keep their fluids up. Good luck
Kindness is the language which the deaf can hear and the blind can see ~Mark Twain