Hum, people say he goes to gym a lot as evidence against steroid use and if this is the case, the likelihood of using steroids would increase IMO. Lot of gym bros just use them without thinking much. It's basically a protein shake for them.
However, dunno how easy is to get them in SK. Sometimes I see in the news people getting caught smuggling steroids as they are legal outside SK, but not in SK. I think you need prescription to use steroids legally. Dunno how deep he would go to break the law for muscles. If it was a gradual muscle gain, I would imagine he didn't use any as getting caught using it illegally could have consequences. He isn't a professional bodybuilder. I don't think male idols are pressured to be muscular. It seems more like a personal thing and he could just take it slow.
That's really not the truth tho, you can't just use steroids on a whim and be nonchalant like that. If you are going to use steroids you have to learn/know the science behind chemistry and cycling the steroids. You also have have to inject it intramuscularly and not just eat it because it'll mess up your liver so bad and cause permanent damage.
Also I only know this because working out is a hobby of mine. I swear I don't use steroids lol.