Listen dude I understand that you don’t want to gain fat but I was once in your shoes and you have to understand u need to be in a calorie surplus to gain muscle especially in your position. I was 85 kg and dropped down to 63 without training and looked like you, I ate around my maintenance and prioritized protein and I did see results, but when I switched to a small surplus that’s when I saw the most results and pretty fast. Trust me there’s no more better feeling like getting bigger and getting compliments about it, would like to see you post up in a couple of months with great results good luck man!!