Accepting diet deficiency/imbalance a quick google search for reasons a dog would spill protein in the urine I came up with this based on OP's health history of his young dog.
"He had bit of issues with ALP in his liver test when he was young. It was 160 or so." [/COLOR]
ALP levels can however be higher than normal in growing young dogs due to bone growth. So this may not be of any significance.
"In december'16 I got his x-ray and scanning done. Mild increase in liver density was shown in x-ray but the scan came out normal"
*Lipemia- basically High Cholesterol can be a cause. High cholesterol can be from to much fat in the diet and the wrong types of fat.
*Hypothyroidism can be a cause of high cholesterol.
*Diabetes can also cause high cholesterol. (This doesn't seem likely based on age.)
*A more benign explanation is strenuous exercise can cause protein in the urine as well. If you are exercising the dog or playing hard before the vet visit this could also explain protein in the urine.
Op you might be looking at the wrong organ. It may not be the kidney's but rather the liver. What color is your dogs poop? Is is greasy looking? Mucousy? Solid? Mushy? Yes yes we really want to know about the poop.
Should the protein spillage be do to lipemia it is likely of dietary origin and easy enough to fix with dietary changes. Since you state he had a scan that was normal you can likely rule out more serious liver diseases at this point.
I feel like such a young dog shouldn't be having these issues.
I got his LFT done too.
ALP, SGPT(ALT) were 151, 44 last time(feb). Now, it is 113 and 70. SGPT increased but still in range. SGOT(AST), Protein, Albumin, Goblulin, A/G ratio were all almost same/normal as earlier. Direct Bilirubin is 0.3. His total Bilirubin was always 0.9-1, since puppyhood, same now.
I got his Cholesterol, Triglys tested in Feb, almost @ lower end of normal. Did not get it checked now.
@ poop: Yup. I understand the importance of poop. It is usually solid with mushy/semi-solid poops coming now and then. Color varies. Usually, yellow to brown. On chicken and rice, it's brown. It he has milk or other light coloured food, it's yellow. No mucous.
Earlier, he used to have bit of grey-ish mixed in his yellow poop. That is gone now. But once a week or so, I do see li'l paleness in some parts of his poop.
I am already giving him liver/kidney cleanser since few months. Will milk thistle help? Don't want to dump him with too much medication.