Is is simply a muscle spasm in the abdominal and has been caused by lifting the heavy weight.
There is nothing o worry and do the following
Ask him not to lift any weight
Do local hot fomentation twice a day
Ask him to refrain from doing any exercise
Give him a anti inflammatory medicine like ibuprofen
in combination with a muscle relaxant
MR,three times a day for 5 days.
If he has some cough
,give some anti tussive syrup.