How can I help my son stop stammering?

Dysfluency is often. temporary in children age 2-5. If it lasts > 6 months, starts after age 3 1/2 or there is a family history of stuttering, seek Speech/Language Therapy. Speak slowly & calmly to your son. Don't interrupt or rush him when he talks. Don't draw attention to his stammer or show irritation. Don't say "Slow Down" or correct him. Therapy includes building confidence to speak.
Depends on age. If kid is 2-3 yo try to ignore it as this is a common transitional stage for some & self corrects. If 4-5 or older you can get help from the "child find " programs operated by schools. In the older child this benefits from professional help. For decades, public schools have had programs for locating & treating kids with speech & language problems.These are available even if they attend private sch.