Fat cannot turn to muscle. They are two different things. The fact that his muscles aren't defined means they're covered by fat. He needs to trim that from his diet until his muscles are defined. If he adds more protein to his diet and keeps at weight lifting he will eventually gain more muscle mass.
@Screenwriter I mean it will get rid of/dissolve the fat. Muscle will replace it. Wrong. The fact that his muscles aren’t defined is because they’re probably tiny since he isn’t usually lifting. If he starts cutting he’ll literally become a stick
@Screenwriter any extra fat will burn as he starts exercising/lifting since there isn’t much of it. If he was fatter I’d recommend cutting but he’s already slim
80% of getting leaner is diet. Only 20% exercise. Adding calories is not the best option. It will take awhile before he burns off that fat. Muscles are bulkier than fat, too, so he won't look "skinny" if he lifts and eats lean protein and veggies. If he loses five pounds of fat and maintains his lean muscle mass, he'll look bigger... It will take longer to gain that five pounds of muscle mass. But he'll get there.
Consult a dietition and look at some of the muscle sites on youtube. See what you think about their recommendations. Your body is unique and you might need to try a few different things before you get best results.
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