I don't think he should have to declare the use of spring attack unless the initial movement would provoke an AoO otherwise.
This will not usually be relevant, however, it will be at times. Consider the following situation: E= Evil Bad Guy, F=evil bad guy's Flunky, P= PC, X= an empty square
Initial Situation
FPF
XEX
XXX
The PC takes a five foot step out of the flank. Ordinarily this will not provoke an AoO from either E or F if it's all the movement the PC does in the round.
New Situation
FXF
PEX
XXX
The PC attacks E once, and drops him. Now the PC might decide "That was a spring attack, I want to get away from the flunkies so I will continue my move." However, in that event, the PC would have provoked AoOs from both flunkies when he took the initial 5 foot step (since that was not the sum total of his movement in a round). Had the Flunkies taken their AoOs when they would have been provoked they might have disarmed or killed the PC or otherwise prevented the attack which dropped the Evil Bad Guy.
If this situation was slightly different, the PC would gain a huge advantage by pretending to merely take a 5 foot step while really spring attacking--consider the following. Here, the abusiveness of the situation becomes even more clear. After dropping the Evil Bad Guy, the PC cleaves into the upper left flunky and drops him as well generating the following situation:
Abusive Situation
XXF
PXX
XXX
Now, the PC is no longer threatened and, if he is not retroactively subjected to AoOs from both flunkies (one of whom is now dead, making the situation even sillier), he will continue his spring attack movement without suffering any AoOs.
Consequently, in order to avoid such situations, I believe it best for characters to declare the use of spring attack whenever it exempts them from AoOs or whenever the first portion of the spring attack is a five foot step.