Or HaChaim on Exodus 18:4/אור החיים על שמות י״ח:ד׳

1 ושם האחד אליעזר, and the name of the one was Eliezer, etc. The reason that Eliezer, Moses' second son is referred to as האחד, "the one," instead of the "second one," is that the reason for Eliezer being named as he was refers to an event which occurred even before Gershom was born. G'd had first saved Moses' life from the sword of Pharaoh before he had enabled him to settle down in Midian.

2The reason the Torah does not mention Eliezer's name first is that it first wanted to stress that Moses was in a strange land at the time the children were born. The Torah retraces events only after the main point that Moses was in a strange land has been established. An alternative explanation, one which I have mentioned on Exodus 2,22, is that when Moses spoke about a "strange land," he referred to our globe, this present life. He did not feel at home in this life, ever. This feeling of being a stranger in a strange land preceded even the time when G'd saved him from the sword of Pharaoh. Moses' whole attitude to life on earth has to be evaluated in that light. This throws a different light on the fact that his life on earth was saved by G'd after he had killed the Egyptian. The Torah had to spell out what precisely G'd saved Moses from or I would have thought that G'd saved Moses from some danger after he had settled in Midian. At any rate, there was something unique about the time when G'd saved Moses' life miraculously.