In this case, the "subject of specialization" was the area in which the person was going to study in college. That is, what subject they would "major" in.
Socks, as you mentioned, are the cloth foot coverings we put on before we don shoes. Many folks only wear a pair of socks one day, then throw them into the laundry pile and put on a fresh pair the next day. So this person changed his area of study the way some people change their socks. One day he was majoring in chemistry, the next day he decided that perhaps art was a better area of study for him.
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