It would definitely be unfair if the absentee gets the same experience with the other players.
Absentee's character may get underpowered, but he still needs to be punished. Otherwise, the other players may get angry with the injustice and the group may see more absent players since there is no punishment for missing the game. Missing a session's worth XP may seem like a harsh punishment, but without any discipline, it will just create a mindset like, "Well, I was busy. But I still get the XP so it should be okay"
The player shouldn't have been absent. It's his fault for missing the game, and if he reacts badly to not being given any XP, it might be the hint of a potential troublemaker player.
There are other ways of gaining XP than killing monsters. If GM gives out bonus XP points for roleplaying, accomplishing tasks, etc; and if the player is dedicated, his character won't be underpowered.
If he isn't okay with receiving no XP because nobody told him about it, then it is time to create a rule which everyone agrees on. If other players are okay with it, he may be given just a slightly lower percentage of the XP but next time, there shouldn't be any pat-on-the-back punishments. Absentees can make it difficult for everyone to enjoy the game and your best bet is a punishment for it.
Also, don't forget to set up a game calendar that everyone agrees on. It's also not fair if the GM just makes up session dates and expects everyone to be free those days.
It seems I sounded like one of those fascistic DMs. Let me clarify. XP is something you should spend effort to earn, so you will feel the sense of accomplishment when you level up. I'm not telling DMs to control their players through fear and intimidation, I'm telling they simply shouldn't reward XP when the player hasn't done anything. Not only it cheapens the value of XP, also not all players will be okay with it when they realize their hard work means nothing. Also, players won't be able to bond if there are arguments about absentees and the game will played with a weird tense atmosphere.
As a reply to the edited question, I would also suggest the XP pool method. It works like this, nobody has an individual XP, all the XP gained is shared by the players and everyone has the same amount of XP. When someone creates an item or casts a XP-consuming spell, the XP is taken out of the pool. So, it turns into a group decision. When somebody gets rewarded for good roleplaying, that XP is added to the pool. It tends to work better in harmonious groups where people know each other well. Efficiency of punishments and methods depend hugely on the players.