In "Word Focus" William Sillings writes:
Verses 22-24 are the crux of this story. The man asked Jesus for help “if you can do anything.” The Greek text includes a small aside at the beginning of this verse (to ei dun), which carries the force of “As to your ‘if you can’ — everything is possible to him that believeth.” It is almost as though Jesus had said, in our vernacular, “If I can! The very idea!” He was saying, “I not only can do something, but I can even do everything you need to have done. The ‘if ’ is not on my part. The ‘if ’ is yours. I can if you can believe. ‘All things are possible to him that believeth.’ ”
The word for believe (pisteuonti) is a present active participle in the dative case, and it means “to the believing one” or “to the one whose defining characteristic is that he believes.” The father answered, “I believe,” in the present active indicative. He was saying, “I do now believe.”
Source: Miracles of Jesus, Adult Teacher's Insights, page 22.