Blunt trauma can lead to cataract formation or central macular edema.... In worst cases it might lead to Retinal or Choroid detachment.
Once cause is known treatment can be done accordingly.
Next Steps
Please don't panic and show him to nearby ophthalmologist to rule out why there is decrease in vision. if required please do B-Scan and OCT of eye if cause is not obvious.
Health Tips
Most of the cases are recoverable if treated on time..