Flat feet is not a problem in such early age, it will resolve on it's own with time. Arch development takes time in some kids.
If your son is having some issues because of flat feet such as pain while walking, running, balance problems etc. then you should consult a physiotherapist.
Next Steps
Don't worry about this if your kid doesn't have any issues with it for now. Give it some time. Let him do all sort of play activities.
Health Tips
You can try cycling and swimming for faster results.