Hey @LimeSqueezer if he hates French then it maybe makes sense for him to plump for the two he hasn't done, German and Spanish, and see which he prefers. But why does he hate French so much? Would he in fact hate studying any MFL do you think? Or is it the teacher, the way it is taught (probs not by a specialist in primary), the challenge of saying it or understanding it? Some of those factors might be an issue with any MFL obvs.
MFL is my subject btw tho I no longer teach it in school. IME one reason a lot of people say Spanish is easier is bc they learned French first (pretty common in England years ago) and then picked up Spanish later - so ofc it was more straightforward as the languages are similar in root and have a good deal of grammar in common. Also people more commonly holiday in Spain from the UK and so are keener to learn the language for that reason.
I don't actually think any MFL GCSE is at all easy, tho I do think there are things about the different languages which may appeal to different people.
All have challenges and easier aspects. German is very logical, with a lot of cases to learn and rules about word order; but it is much easier for an English speaker to get decent at pronunciation. French and Spanish much harder for us to say; the j in Spanish is especially challenging for a UK tongue to master. This may also lead to issues with listening tasks. In French and Spanish you have to make adjectives agree with the gender of the noun. I could pick examples galore - natation in French is the same as natación in Spanish is the same as Schwimmen in German - hmmm the German word is easier there! But I could pick examples the other way too for sure.
FWIW wrt grade boundaries - they are pretty similar. In 2019 (last reasonable year to look at really) there were just a few marks difference. 135 for a 6 in German, 138 in Spanish, 141 in French; 201 for a 9 in German, 205 Spanish, 203 French. That's out of 240 total btw.
Could he try some duolingo on the two he hasn't tried and see if he likes them better? If you are worried about him losing the learning of French in primary if he drops it, I wouldn't; any secondary school will start everyone from scratch and IME so little it learned at primary level sadly that is makes no difference. HTH.