You are going to need to find out what his requirements are. What music does he want to play? Has he previously owned/played one? They come in a huge variety of shapes/sizes/cost. Assuming we are talking piano accordion (the one with a piano keyboard for the right hand – there are other types), the main differences will be:
Number of bass buttons – generally 48 – 120
Number of “voices” – each has an individual set of reeds.
The more of each the bigger, heavier, more expensive, but more versatile the accordion becomes.
Have a look at Hobgoblin or Accordion Shop, you’ll see what I mean.
That Hohner Verdi helpfully pictured above is one of the big uns.