I disagree with Alex. The verb "is" that goes with Tom and ought to be singular is the second word of the sentence, "Tom is...". The main verb of the sentence is already conjugated and puts no restriction on the second "to be".
The word in space can be either "is" or "are", depending on whether the "who" refers to Tom or to "the boys". Both can be correct, but the meaning of the sentence would be different:
"Tom is one of the boys who is fond of sports."
Among "the boys", Tom is the one who is fond of sports, and "who" here refers to Tom. The other boys might or might not be fond of sports. The focus is on Tom.
"Tom is one of the boys who are fond of sports."
Here "who" refers to "the boys", in a sentence structure "Subject is one of X", where X is a compound phrase "the boys who are fond of sports". Tom is one among the group of boys whom all are fond of sports. Tom is also the subject, but a part of the focus is also on "the boys". Notice the difference?
In both cases Tom is the subject, but "who" refers to Tom when "is" is used, and "who" refers to "the boys" when "are" is used.
If "is" is used, we don't know if the other boys are fond of sports or not. But if "are" is used, we know for sure that all the boys, including Tom, are fond of sports.
Check these other examples:
"Obviously Pliskova is one of the players who has the perfect game for grass." Metro
Piskova is one of... who has (singular)... Full focus on Piskova
"Mary is one of the students who have done honor to the college." University of Wisconsin Madison
Mary is one of... who have (plural)... Some focus also on "the students"
Hope it helps!