Madrid to Malaga - time in between?

Zero time for customs unless he has something to declare. Immigration has taken as little as 20 minutes and as long as 1.5 hours. Then of course is the time needed to get off the plane and walk to passport control. Usually, any checked baggage is ready for pickup by the time one clears immigration.

If he’s taking a train to Malaga then he needs to get to the train station.

Just a carry on from JFK, nothing to declare - but new airline/ticket for Iberia to Malaga. Arrives at 10:10, wanted to put him on an 11:30 to Malaga. Seems tight?

I don’t recall how long it took the last time I had a connecting flight in Madrid (2011) or whether there was a separate immigration line for those with connecting flights, but as long as the flight arrives on time, he should be fine. Only having a carry-on will help.

Personally, I don’t like tight connections because I’ve had too many flights depart late. Back in 2006 we were literally running to catch our connecting flight from Madrid to Barcelona and just made it before the airline closed the aircraft doors.

I am someone who likes to make my layovers as tight as possible, but I would not count on making it with that tight of a turnaround on a ticket that was booked together, the fact that they are separate tickets makes this pretty risky. He might luck out, he might end up in a long line, which I experienced at MAD the last time I was there.
Is it too late to change the flight--was the flight to Malaga that much more expensive than Madrid? Sometimes it is worth the extra cost so you don't have to sit around in an airport. He could also go directly to the train, which may be a wash time-wise.