Jesus gives no indication that the Father has no form. He is saying here that He does indeed have a form. He teaches that He has a voice and shape.
The word "form" here is from the Greek eidos, which means, "form, shape, appearance, fashion." It is used in a context indicating what can be seen with the eye. That is what Jesus says, "You have not seen His form"—that is, with the eyes. He does not mean something visualized in the mind.
Luke 3:22 says, "And the Holy Spirit descended in bodily form." Those who were there saw it with their eyes, and it had shape to it. Likewise, the Father has shape that is visible to the eyes and a voice that is audible to the ears.
John W. Ritenbaugh
Image and Likeness of God (Part One)
Who is Jesus addressing in this context? To whom does He refer as "You"? Verses 16 and 18 give the answer:
For this reason the Jews persecuted Jesus, and sought to kill Him, because He had done these things on the Sabbath. . . . Therefore the Jews sought all the more to kill Him, because He not only broke the Sabbath, but also said that God was His Father, making Himself equal with God.
Jesus is not talking to or about mankind in general. Nor is He talking to His disciples, friends, and followers, though undoubtedly some of them are with Him. He is addressing a gathering of Jews, some of whom are trying to kill Him because He had called God His Father.
Throughout His discourse, Jesus is actually introducing and revealing God the Father to them. The Jews had not known God the Father previously. They and their forefathers knew Yahweh to some limited extent, but not God the Father. All of their dealings with God had been through the Logos, who became Jesus Christ. They had never seen the Father, and they had never heard His voice.
The Jews had had no experience with the Father. If they would believe the Son's words, however, they could have a relationship with the Father (John 5:38; 8:19; 14:6-7,20-23; 16:27; 17:20-26).
The Voice of God
Other Forerunner Commentary entries containing John 5:37: