1) Did you check his shoes that he wears? Is the shoes size way too big or too much exceeded space inside?? If so, that may cause him fall every time when run. The shoe shouldnt have too much space inside more than a thumb.
2) Check see if his walking pattern. If he is not walking probably that might cause him tripped over on his own legs when he runs.
3) If not cause by above, then he can play more 平衡木 that type of things to train the balancing. Or you can use a masking type typed on floor and let him walk on it at home.
4) Or put several cusions on floor to let him walk on it, that also train his balancing.