You would lose about 50fps per inch. This would shorten its effective distance. Greater bullet drop at longer ranges ect.
It would be a bit handier for carry in the field. These are not really ment for off-hand shooting. More for using a bi-pod or rest.
If you are only punching paper, leave it alone. Your son will get stronger and be able to lug it to the range. If it is a hunting rifle,
then figgure what the max range you would be shooting it and what kind of game you are going after. There are bullet drop tables out there
for the different .308 loads that will let you know effective range. If you deside to have the barrel cut, have a good gunsmith do it.
Have a 11 degree recessed crown done to it.