Mandarin may be very fashionable, (my DSs had a little taster and also found it fun) but it is very difficult,and so only choose it if:
(a) you are certain he will have a good teacher (and good luck to you trying to establish that)
(b) he can drop it before GCSE, or you don't mind a lower grade because it probably will be, because it is very difficult, which is why few westerners speak it.
Look at prior year results on the school's website, and subtract any native speakers to get a fair picture. If he can drop it later , put it as second choice.
Spanish has now overtaken German as the 2nd language after French at schools. It is easier than German (I have studied both) and as TIPSY points out it is widely spoken, for example it is now the number 2 language in the USA, appearing in Ads , official documents etc.
I would say Spanish first, Mandarin second, unless he can't drop it, otherwise French because he already has some, and he won't like German any better.
Nothing like conflicting evidence is there
. Full disclosure of bias: I speak French and some Spanish.