Even with a tiny change in relative proportions within transport, road vehicles still use vastly more fuel than aviation in the UK. Without absolute tonnage of emissions, and relative proportion of commercial and private, I can't see a way to invalidate his claim or prefer aviation as culprit. It sounds perfectly reasonable and the least controversial assumption from data available: cars are the big majority of the largest fuel user, though commercial traffic may get low enough mpg to emit disproportionally large amounts.