What exactly does he mean?
He means he calls anything he doesn't understand a ponzi scheme.
Bitcoin may be many things. It may be horribly flawed, it may be naively optimistic, it may never gain more than a marginal niche, hell it may crash and burn spectacularly tomorrow but it certainly isn't a ponzi scheme.
Let him know the ECB (no friend to Bitcoin) said as much in their report on virtual currencies. Actually no regulator, governmental agency, or consumer watchdog group has stated Bitcoin is a ponzi.