With a son in competitive tennis, you may be interested in reading the USTA Recovery in Tennis booklet - it is an easy read and has many practical recommendations:
USTA Recovery in Tennis http://assets.usta.com/assets/1/dps/...%20VERSION.pdf
For instance the following advice on when to return after play is found there, along with a pain scale:
"A pain rating scale can help determine the
severity of an athlete’s situation and his/
her participation capabilities for upcoming
training or competition. An athlete should not
continue training or competing if swelling,
range of motion limitations (above normal) or
muscle weakness (above normal) is present.
The pain scale can also be used to assess
the degree of recovery from any type of
injury. Return to play may be allowed if pain
levels are 1 or 2."
It may seem quite "unscientific" to rely so much on pain as an indication on when to return to play, but outside of being evaluated medically it is probably the best indication we have.
But as your son's case illustrates, tennis can suddenly place undue stress in the most unexpected manner. The sudden movements that caused both the initial injury and the recurrence don't always occur during the course of a match - but are not all that unusual either.
It is what makes tennis a tough sport.
Is your son doing off court conditioning with special emphasis on the core/back?