yes £240 on £10k and £480 on £20k unless if the interest is applied yearly, if invested for more years then he will also accrue interest on the interest over the following years. Therefore, long term, the more he puts in the greater the interest and the highet the eturn on the larger amount.
It depends on the terms and conditions of the ISA. Some cap the amount on which their top interest rate is paid, which in this case may be £10,000. You need to examine the T&Cs.