Has he already studied Latin? If he would be starting it from scratch that's more of a high risk than taking History. Signs that he could do well in it: being good at modern languages, having a good memory, having an analytical mind, being prepared to work hard and stick at it even if he finds it difficult sometimes; and (very, very important, this bit) really wanting to do it, not just taking it because the other options are less appealing or because his friend is taking it.
I studied Latin and Ancient Greek at university, having had the great good fortune to be able to do both at school. My daughter is following in the same path, although she had to do Greek GCSE outside school. For both of us we just knew from early on, goodness knows why, that these were subjects we passionately wanted to study and neither of us has regretted the choice.
There are -ill-informed- people who can see no merit in studying Latin because it's a 'dead' language, ie nobody speaks it now. This is a silly argument.
Firstly, it helps with learning other languages. Spanish, Italian, French and Romanian all developed from Latin after the Roman Empire collapsed, so a lot of the vocabulary comes from Latin.
Secondly, studying Latin is a huge boost to a person's English vocabulary because so many of the more complex words we use have been borrowed from Latin. It helps with spelling those words as well as learning the meanings.
Thirdly, it's a really good way of developing your ability to think logically. The grammar of Latin works differently from most modern European languages, so to translate it into English you have to learn to think in a different way. You need to learn the grammar rules and lots of vocabulary. There's a crossover with maths and computer programming here, which many employers will recognise.
And finally, from the point of view of literature, culture, general knowledge, history etc etc, the Romans were hugely important and their legacy lives on. Some of the world's great classics were written in Latin, and there are countless references to Latin literature and Roman mythology in English and other European literary traditions. Studying Latin will involve learning more about Roman civilisation which will be interesting and useful.
Having said all of that, he can't go wrong with either Latin or History. What a shame the options system doesn't let him do both. Good luck to him!