In English "someone" or "somebody" is referred to with "they" even though it is singular. This is because it is of unspecified gender. What is the convention for referring to хтось (or a person of unspecified gender) in Ukrainian?
The convention is to refer to "хтось" as to "він" (he): singular masculine gender.
As we can see from examples in СУМ:
— Увійди та одпочинь, дитино, — промовив хтось тихо й поважно (Марко Вовчок, I, 1955, 10);
Од них ясніє дивно путь, неначе струни на гітарі перебирав ніжний хтось... (Володимир Сосюра, I, 1957, 410);
It would sound odd if you use any other grammatical gender or number.
Update: When you refer to "someone" and you are pretty sure that there is more than one person, or it is unknown whether the word is used for one person or several, it is ok to use "they". But when you are sure that there is one person and you don't know the gender, you should use "він" (he). When you know the gender, you most probably shouldn't use "хтось" at all. Several examples.
Here, we are sure that there are several people and use "they":
Через стіну було чути голоси - хтось святкував день народження і, здається, їм було байдуже, що завтра іспит.
(You could hear it through the wall that someone had a birthday party, and they didn't care that the exam was the day after.)
Here, we have no idea whether there was one person or several people. In this case it is OK to use both "він" ("he") and "вони" ("they"):
Хтось прибрав у подвір'ї. Які вони молодці!
Someone has cleaned up the yard. That's very nice of them!