If the CPS, Crank position sensor, is something other than Bosch it has been found through experience that it often fails prematurely: Thus it may be the cause of your starting problems. You should pull it out and check to see if it is something other than a Bosch. Get a Bosch if it isn't.
ala-xfire was thinking you had low fuel pressure. He thought by running the fuel pump three times before starting it would build pressure in the fuel rail up at the engine. The fuel pump does not run at the second auxilary position at the key. You must turn the key to the third position were all the lights on the dash turn on to run the fuel pump. It would be good to be certain that third position is the one your son used when he attempted the fuel pressure build up and not just the auxiliary second position on the key. There is a check valve/regulator in the fuel filter, or the pump itself may be weak. They are located just forward of the right rear wheel under a plastic shield.
The water in the trunk can cause electrical problems and lead to expensive repairs if not corrected early. As mentioned, the air pump that also creates vacuum to lock/unlock the doors and a few other electric components are located there. Water cause electrical problems and possibly is causing the car to think someone is tampering with the car, so the anti theft device is activating and thus the SKREEM is doing what it is suppose to do and preventing the car from starting. Probably the easiest way to check for water back there is to get under the rear of the car on the passenger side. You probably will have to raise the car some to see what you are doing. There are two rubber plugs about one inch in diameter and they are covered with undercoating. The plug are aft of the axle. Use a small pocket knife or an ice pick and make a hole in the plug nearest to the exhaust pipe (there is nothing above it to damage). Twist the ice pick or knife a little and see if there is water leaking out.
Using the search box or searching the threads in the link I gave you will give more detail on anything I have said above, and be certain to check out the starting issues in the service manual it will step by step lay out how to check the starting system and the pulse module. The other common starting problem area is the RCM, relay control module. They have several contacts that get burned contacts and start causing starting problems. Look up RCM in the search box. There is a member in the forum that repairs your RCM for about $40 . Many of us keep a spare RCM in the glove box or trunk as it is a common failure.
If you choose to get the car scanned, use an independent garage that specializes in European automobiles, not a Mercedes dealer and certainly NOT a Chrysler dealer.

Generally buying parts for these cars is best done online from Mercedes or independent parts dealers who will get you the correct part. FCP Eur is just one of several. The parts sold at Advance auto, Auto Zone etc. are cheap china made and can lead to trouble. There are plenty who disagree; Take it for what you paid for it, but that is my advice.