Junior cassette would work best. You will have to scratch around a bit for them, but the Shimano makes them in 10 and 11 speed, as do others.
If you want to have and adult gear a 12T cassette might just mean blocking the two cogs - but you are still buying a new cassette.
This is very equipment specific. I think for a TT what you are doing is also ideal for chain alignment - if he can push the 14T.
Some setups you can replace the cogs with spacers. That may be more of a pain - and he can shift off - and on again, but a pain.