Sound   Noise
Level
(dB)   Effect      
Jet engine (near)    140    
Firecracker    140    
Gun firing    140    
Rock concert    110-140    
Boom box    120    Threshold of pain (125 db).
Thunderclap (near)    120    
Stereo (over 100 watts)    110-125    
Chainsaw    110    
Jackhammer    110    
Snowmobile    105    
Airplane (near)    103    
Garbage truck    100    Regular exposure of more than 1 minute risks permanent hearing loss (above 100 db).
Farm tractor    98    
Motorcycle (near)    90    No more than 15 minutes of unprotected exposure recommended.
Subway    88    
Lawnmower    85-90    Level at which hearing damage begins (85 db).
Television    70-90    
City traffic noise    80    Level at which constant exposure may cause hearing damage.
Alarm Clock    80    
Washing machine    78    
Dishwasher    75    Long exposure to sounds less than 75 db is unlikely to harm ears
Vacuum cleaner    70    
Hair dryer    70    
Laughter    60-65    
Normal conversation    50-65    
Refrigerator humming    40    
Quiet room    40    
Whisper    30    
Rustling leaves    20 
OK those are some normal everyday sounds here are some airsoft pistols loudness
Springer - TM S&W PC356    84dB
Maruzen S&W M4506    88dB
Marushin Bren Ten 8mm    86dB
Tanaka S&W M29 6.5"    84dB
KSC H&K Mk23 SOCOM    94dB (86dB with silencer)
Western Arms SV Infinity Ltd 6"    96dB
Western Arms Beretta Cougar 8045    96dB
S&W Performance Center M945    94dB
KSC M93R II    94dB
So I personally dont think it was teh loudness of the pistol but when the gun was fired he was near the ports that release the air pressure. That in turn ruptured or tore his eardrum. I personally would go get checked out for that possibility. Check out this site it will explain what I mean.... 
http://www.nlm.nih.gov/medlineplus/ency/article/001038.htmNow I may not be a doctor but going by what you said I did a little investigation and my opinion is to go get checked out.