Excessive urination. This query is!

This query is regarding my son age 5.5, weight-26kgs. He has autism and has limited communication. Since last 3-4 months, he was informing us whenever he had the urge to pee or would go to the bathroom himself. However, since 10 days, he is urinating a lot and mostly in his pants. It started with peeing once in his pants for three days, then he started peeing every 10 minutes and woke up in the night and urinated 3-4 times for 2-3 days. Now the frequency has come down and it happens only after drinking water now. He is still peeinh every 20 minutes after drinking water, in his pants unless we ask him to use the bathroom at regular intervals. And once after waking up in the night. He is not drinking excessive water, infact a little less than earlier due to cool weather. He is otherwise fine, doesn't feel uncomfortable while peeing, urine is clear and the urine output is normal too. Can you please advise what could be causing this?