As far as I can recall, compulsory MFL at primary level only comes in this September, so the school cannot assume prior knowledge of any of the languages. Presumably AGSB will be familiar with what is and isn't taught at local primary schools? However, indies would almost certainly teach at least French at KS2, so if the school has a significant intake from outside the state sectorhe may feel a little behind initially, but I wouldn't worry too much about this, personally.
If you do want to give him a bit of a grounding before he starts, you could look out for holiday courses at language schools, if you have any locally. Or have a look in a bookshop or your local library - I would suggest getting something with an accompanying CD, given that French (unlike German) pronunciation isn't quite 'see and say' to non-French speakers, IYSWIM