“an invalid” probably indicates he had paralysis since he had a bed and was incapable of being first into the water unless someone would help him.

John probably chose this particular sign among many to affirm the messiahship of Jesus in accordance with messianic prophecy in the Old Testament in Isaiah 35:6.

Isaiah 35:6  Then will the lame leap like a deer, and the mute tongue shout for joy. Water will gush forth in the wilderness and streams in the desert.