Surgery seems very extreme - although his choice if that's the only way he wants to proceed.
Can he not try to 'work on it' and try to adjust the pitch deliberately - maybe with the use of a voice recorder; try to expand his 'range' as it were, like a singer might do, and then favour using the lower end of it? I'm sure this would be hard work at first, but would then become second nature as time goes on.
A lot of trans people deliberately lower or raise their voices without hormones or surgery, in a desire to 'pass' better as the opposite sex.
It is difficult, though, and some people are just naturally outside the 'norm' for it. I used to work in a call centre with a man called 'Tim' (let's say) and people would ask his name for reference at the end of the call. The number of people who would say "Tim? That's an unusual name for a woman, isn't it?!" and it would clearly never cross their minds that they had probably been talking to a man (hence the name 'Tim') with a feminine-sounding voice. We also used to know somebody whose mother had a deeper, gruffer voice than just about every man we know.
For some reason, I also recall this story from years ago, where a man was effectively locked out of his own account when using telephone banking. I would have thought a lot of people would find themselves on very dodgy ground nowadays, though, insisting that somebody with a particular voice, name or look cannot possibly be what they identify as, as so very many people identify as the 'gender' associated with the opposite sex to themselves, and many deliberately make no physical or social changes to help them 'pass' e.g. transwomen maintaining full beards, transmen choosing to get pregnant.
www.manchestereveningnews.co.uk/news/greater-manchester-news/bank-thought-man-was-a-woman-1004309