Personally I’d go for French and Spanish, or French and German. Spanish is valuable particularly bevause of its use in Latin America and Caribbean with the former being a particularly important place for business. Also I know companies expanding into Spain now. It is very commonly taught around the world and is valuable for its literature, history and travel opportunities. Many people never use any language they learn at school outside of travel tbh, and Spanish opens up an entire continent (barring Brazil) + Central American countries and Mexico. These places are incredibly varied and exciting to visit, and many have regions where people speak relatively poor English, so it’s particularly useful to learn. I didn’t learn Spanish at school but grew up with a mum who spoke a language which is is mostly intelligible with Spanish (I am nowhere near fluent and only know basic conversation) but that has helped me. If they ever want to work in the US it is the most valuable second language there. He will have an easier time with his French and Latin background - and his Spanish can help him in French and Latin too.
French is really valuable for anything in Europe, in the UK it’s usually seen as the default second language to learn. It opens up a lot of West and North Africa, parts of the Middle East, bits of the Caribbean, parts of Canada, etc. It is therefore great for travel, but also for business, research, meeting new people etc. There are important francophone universities he could benefit from, as well as their research, if he wanted to in the future. He has already learnt a little and making sure that doesn’t go to waste and building off it would be great.
With that said, German is definitely an important language for business - probably the second language for European business etc, plus its good for travel, universities, research, literature and culture etc. Not to mention, if your DS learns a Latin based language now, and later decides he wants to learn say Spanish for work or travel, he would find it easier to start as an adult. Learning German now won’t stop him learning a Romance language in the future after all, and he’ll probably find it easier to start learning one of them than German.
Despite mentioning stuff like business, most people who study a language at yr7 are not going to take it up to A-Level which is probably the minimum for when it is useful for career opportunity, if they stop aged GCSE they’ll likely forget almost all of it or it will be at a level which it isn’t too difficult to work up to with more concentrated study as an adult (I’m learning a language now and that’s my experience for now, but maybe I just had poor language teaching in school). While it’s inportsnt to consider the future and set the stage for taking a language further if they want to, they’ll probably be using this language for travel, and a dedicated pupil might want an exchange trip and to consume online content not set as homework. Mandarin is going to see little use because getting to a level of being able to read basic information in Mandarin is very hard! Unless you’re travelling to China regularly, they’ll not use it for travel. In comparison, French and Spanish will likely be the easiest in terms of going on any holiday in the future as it’s often just as cheap to fly to Spain as it is to stay in the U.K., and easy to take weekend trips to France and Spain, there’s also opportunity for visiting Belgium, Switzerland etc. German is also useful for travel to Germany, Austria and Switzerland of course too. Unless your son learns to speak mandarin extremely well, the time is unlikely to pay off, especially if he’s only going up to GCSE (which is quite a low level language wise), whereas German, Spanish and French can have that use quite easily. Id go for Spanish and French just due to ease of learning them together, and having the widest opportunity for travel and the expanding markets in Africa and Latin America, plus the widespread and diverse content/media, countries to use it in etc.