I decided to resurrect this thread because I have exactly the same situation.Before you start cutting drywall there are a few things you can do to try to narrow your choices.
First, make very sure that the water is not just running down the corner of the tub past the shower curtain while your son is taking a shower. Take a very close look at the wall and floor corners at both ends of the tub immediately after he is finished. Pay special attention to the end the shower head is on. I have seen many "leaks" that wound up being caused by water running over the edge of the tub. Enough people have this problem that you can but special little dams that stick in the corner on the tub to block the water.
If you are sure this isn't the problem the next step is to determine whether the leak is from the valve or from the drain. Since it doesn't leak all the time, it's not from the hot or cold inlet connections. These are under constant pressure and would leak continuously if they were the problem.
The easiest thing to check first is the drain side. You don't want to run the faucet because that will screw up this test. Fill a bucket up from another bathroom and use this to fill the tub. I know that's a pain in the neck but it's necessary. Once the tub is full let it sit for 30 minutes or so and see if the leak shows up. If so, it is from the tub drain. Drain the water and fix the drain. It probably just needs to be removed and re puttied. Clear silicone, though harder to clean up, works great instead of putty.
If no leak shows up while the tub is full go ahead and drain the tub, again checking for leaks. If you see a leak now the problem is in the drain piping. You will need to cut drywall to find and repair this. Start at the wet spot.
If no leak shows up from the first test, that leaves the tub spout piping and the shower head riser and connection. The two most likely places to check for a leak are the shower riser and connection and the tub spout piping and connection. The first thing to do is to pull the escutcheon (trim ring) at the shower head and see if you can see the connection inside the wall. You may have to open the hole a little, just enough so you can see but it will still be covered by the escutcheon. Now run the shower head while looking at the connection with a flashlight. If you can't see it very good, wrap some toilet paper around the end of a screwdriver and hold it under the connection for a minute. Check and see if the paper is wet. If so, or if you can see a leak, you've found the problem. Take the shower head off, stick the handle of your pliers in the end of the bent pipe coming out of the wall and remove it. Put some new teflon tape on the threads, screw it back in and check for leaks.
If you don't find a leak here check the tub spout. If you have the kind of tub spout with a diverter you have to pull to make the water go to the shower head it could be leaking where it connects to the piping and water could be running back into the wall. This is kind of hard to see but some brands have a small opening on the bottom side of the spout to tighten the spout clamp. You can use the toilet paper trick here too. If there is no opening you can try to unscrew the spout and see if the piping behind it is wet. If none of this works you may need to cut drywall behind the tub to really check it out (if you can get to it).
If you can find the source of the leak this way you may be able to fix it yourself and save some money. Even if you don't feel comfortable doing this, you will at least be able to let the plumber know what you've done and minimize the cost of the repair.
However, I have narrowed down the leak quite a bit now and am looking for advice on how to actually fix it.
For a long time I believed the leak was at the place where the shower neck meets the shower riser pipe elbow inside the wall. However I've replaced the neck with a new one, used a lot of teflon tape and nothing changed. Drip, drip, drip.
The drip comes when either the shower is running or there is water in the riser pipe (the shower/faucet handle is set on shower). So, my only remaining guess is that the leak is where the elbow connects with the riser pipe, because it is only when there is water in the column that the dripping continues. Is there another explanation?
And, if so, what do I need to do? Unfortunately I don't even have drywall on the wall that covers the piping - I believe it's plywood. And, I believe the problem to be more than just a leaky valve or washer because I know that the previous owners had this leak, since it was clear once the leak started again that the ceiling had been patched in exactly the same place before.