Children of this age can show changes in their patterns of passing urine .. as they are still in the process of getting toilet trained . Check for constipation . Increased frequency can also be because of associated urine infection .. you have to note wether the child is uncomfortable while passing urine or has fever .

It is fairly normal for a child of his age to have altered urine pattern owning to lack of proper bladder training or sometimes even due to cold temperature, however there could be issues like metabolic issues or urinary infections that might be missed.. the final decision would depend on the co existing factors like the child's growth, other urinary symptoms / presence or absence of fever etc. So ideally get in touch with a paediatrician and discuss in detail to get the further plan of action.