It is your decision if you do not want to include your son in your PR application. However, it is recommended not to exclude any member of your family while applying. The Singapore government sees your children as possible future contributors to the nation’s economy and society.
The basic rule behind the approval of PR application is are you ready to sink your roots here in Singapore if given PR status. If you do not include your son in your PR application, then your chances of successful approval will reduce.