Maybe have him take some math practice tests - use a timer - and see how he does. My d did sections of the SAT (25-30 minutes) each day for several weeks. Hope he has a clear calendar next week
Maybe have a look at the schools he's considering who offer merit aid for 1400+ .... do they even require subject tests? In general I would say most schools are more interested in the reasoning than the subject tests. But that's a generalization