Thanks for your query,
Crying in newborn can be due to so many factors;it is their way of communication.they cry due to;
-Any underlying disease process.
Since baby is gaining adequate weight so do not worry.
Do burping after each feed for 10 to 15 minutes.
Keep baby in prone position but baby should not be left unattended.
In case this persist get him evaluated by pediatrician
Wishing your grand son good health.