Jack Simth said:
It can be stopped, easy.
He can't do it that way; he can't get constant coverage from the spell.
You see, Timeless body also prevents helpful effects ... which Timeless Body, in and of itself, is. As the TB doesn't expire until the END of his turn, he can't manifest it again on his turn (well, he can, it just won't affect him, and will expire when he doesn't expect it to).
Okay, he can maybe ready an action to manifest it after it expires.... but then there's a forced gap where he isn't protected, and he's subject to concentration checks from things like damage due to a readied action to disrupt spellcasting, continual damage, casting defensively, and what have you.
He doesn't need to Ready an action. As stated by the OP, round one he casts ETB, round two he does something else, round three he casts ETB, round four he does something else.
The only window is just before (in this example) the odd rounds (starting with round three since unless the NPCs are aware of his tactic, it would be a bit of DM metagaming for the DM to ready actions against him in round one). However, his opponents would be wasting readied actions just before the even rounds on him. They could only sneak in with readied actions on odd rounds and quite frankly, if they are sitting around readying a lot, they are not really fighting. He could easily manifest powers on the even rounds which do battlefield control, etc. and pretty much mop up the combat. Sure, every once in a while, an opponent might disrupt this tactic on the even rounds with a Readied Action. But, that will only happen if the Psion does not take charge of the situation, especially in round two.
Also, some other powers can really limit what happens to him in the tiny window immediately before his odd rounds. For example, a multi-round Intellect Fortress cast first in round one will not help him while he is protected by Timeless Body, but it will protect him in the tiny "ready" windows. A 20th level Psion could put this up for 14 rounds. 24 rounds if he could expend two psionic focuses in round one (one for Extend Intellect Fortress, one for Extend Timeless Body).
Plus, most Psions I have seen played have had multiple buffs up a lot of the time. So, in the tiny windows, his buffs will come back up.
But, there are some minor downsides to Timeless Body as well. For example, Body Equilibrium will not work, Psionic Darkvision

will not work (if the party moves around with Darkvision from spells and powers, it will be interesting the first time the PC tries the tactic and he suddenly cannot see in a cavern or some such), Schism will not work, Boost will not work, etc. Any of the psion's buffs will not work. So, any attack by the Psion will often have a lowered DC (if his intellect is boosted) or will do less damage (for a melee attack if his strength is boosted) or will have a lesser chance to hit (for any attack that uses Dex with an attack roll), etc. And, without buffs, he can probably be more easily grappled as well.
If his Intelligence is boosted, he will lose some PP when EBT drops it and maybe not get them back (depending on how the DM rules this, I'm not sure if there is an official rule about ability loss and PP).
And, he will have to use up his Psionic Focus every other round in order to maintain the EBT defense. He can only use his Speed of Thought, Psionic Dodge, Up the Walls feat or other must be psionically focused feat every other round.
All in all, Extend Timeless Body is a pretty cool high level tactic which would force opponents to get real creative to get around it (e.g. Readied Actions, AMF, grappling, etc.). And, most opponents seeing it for the first time should not be experts at defeating it. For example, if an NPC Readied Action fails on round two, why would he try another Readied Action on round three?