。
I owe him considerably.
Literal
I [topic-は] him [to-に] not-a-few obligation [subject-が] exist.
少なからず ('not a little, considerable') is a literary negative-to-positive expression. 恩義 ('obligation, moral debt, debt of gratitude') — a key concept in Japanese social relationships, reflecting the importance of reciprocity and repaying kindness. に marks the person to whom one owes.