My son is almost 5 years old and from today he has been urinating every 30 minutes. i am really concerned. what can cause this?

There are many things which can do that. Make sure he is not constipated. Avoid carbonated drinks and see if the urinary opening is clean. If it persists then he has to be seen by a doc/urologist

If he's voiding frequently and in small amounts, it's most likely a bladder,urethral or urologic issue. If in large amounts and thirsty, an endocrine or metabolic disorder like diabetes mellitus or insipidus, serum calcium disorder would be more likely.