Your son will need a UK licence, or foreign licence equivalent to the UK licence.
After 12 months in the UK, he’ll need the UK licence.
“AM”-category Aixams do exist in the UK, but are rare because they are beneficial only to 16-year-olds - after that, they can get (and will want) a proper car and full car licence.
Insurance is surprisingly expensive for a young person in an Aixam (think £2000 per year, but that depends on age and driving experience). The maximum allowed speed of 28 mph (45 kilometres per hour) can cause problems - other drivers will not be as forgiving in Liverpool as they would be in rural France!