This is a really GREAT question that doesn't come up often. The World Pool-Billiard Association are the de-facto "base" set of rules which most leagues and organizations base their rules off of. There's nothing specific in there to address this scenario.
The consensus among referees I've asked about this question is:
You can't call both (a) a ball and pocket, and (b) also call "safety".
So in that case, the order in which those two things were called is important. Whichever call was made last (or, most recently) is the one which applies. It's like changing your mind. Your latest call must be taken as the intended call.
Given that, we can say that the "safety" call is what counts in your scenario. The way you explained it indicates he called the ball and pocket first, then called "safety", and so it's the "safety" call that applies.
Thus, the table remains open after his shot, and play passes to his opponent.
That being said, if you find any specific rule reference that disagrees with what is written here, please send along. I would be interested in seeing it!
Here are the World Pool-Billiard Association's rules for 8 ball which would apply here.
Rule 3.4 about open table and choice of group tells us that during an open table, groups are chosen only on a legally pocketed ball (not a legally pocketed shot).
3.4 - Open Table / Choosing Groups
Before groups are determined, the table is said to be “open,” and before each shot, the shooter must call his intended ball. If the shooter legally pockets his called ball, the corresponding group becomes his, and his opponent is assigned the other group. If he fails to legally pocket his called ball, the table remains open and play passes to the other player. When the table is “open”, any object ball may be struck first except the eight ball.
Rule 3.6 "shots required to be called" tells us that a player can either call a ball and pocket, or call a "safety". It does not say a player may also call a "safety". The absence of the word "also" indicates that it's a one-or-the-other call.
3.6 - Shots Required to Be Called
On each shot except the break, shots must be called as explained in 1.6 Standard Call Shot. The eight ball may be called only after the shot on which the shooter’s group has been cleared from the table. The shooter may call “safety” in which case play passes to the opponent at the end of the shot and any object ball pocketed on the safety remains pocketed. (See 8.17 Safety Shot.)
Furthermore, rule 8.17 which defines the safety shot indicates that the only outcome of a called safety is that play passes to the opponent at the end of the shot.
8.17 - Safety Shot
A shot is said to be a safety shot if the game in play is a call shot game and the shooter declared the shot to the referee or his opponent to be a “safety” before the shot. Play passes to the other player at the end of a safety shot.
The only set of rules I've seen which address this somewhat specifically are older BCA rules.
4.16 - Illegally Pocketed Balls
An object ball is considered to be illegally pocketed when:
- that object ball is pocketed on the same shot a foul is committed, or
- the called ball did not go in the designated pocket, or
- a safety is called prior to the shot.
Illegally pocketed balls remain pocketed and are scored in favor of the shooter controlling that specific group of balls, solids or stripes.
Since BCA rules also say that groups are only chosen on a legally pocketed ball, rule 4.16 supports the call that the table is still open.