A warm bath helps the cramping of the bladder and relaxes the muscles, if you think he would be able to relax in the bath. Is he drinking enough liquids to pee? Usually with a bladder infection the urge to pee is so strong that it completely empties the bladder.
Try offering a liquid that he is fond of to get more in him. Sometimes a warm pad or towel held on the bladder will relax the spasms too.
So sorry for your little guy. Good luck.
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