In the hospital, I met a six-year-old boy named Xavier. He was being treated for leukemia.

In the hospital, I met a six-year-old boy named Xavier, he was being treated for leukemia.

In the hospital, I met a six-year-old boy named Xavier, who was being treated for leukemia.

The second sentence combines the choppy sentences by making the second half of the sentence a subordinate clause. The other sentence contains a comma splice (two independent clauses joined with a comma and no coordinating conjunction such as and).