This may just be a normal part of development, especially if your son is trying to walk further or faster, or to do activities which require higher amounts of balance and coordination. It could also be a sign that their footwear is not fitting correctly, or is not securely fastened, so this is definitely worth reviewing. In less common cases, this could be a sign of problems such as muscle weakness, or neurological conditions. If you cannot identify a more likely cause such as those suggested here, it may be worth speaking to a childhood physician or physiotherapist to explore further.