First check for a UTI - that can make the bladder sensitive and prone to involuntary emptying. If no UTI there are medicines that can help. If meds don’t help, you can buy/make belly bands to catch the urine and keep his bed clean and dry while he’s asleep. I put a thick soft disposable incontinence pad in my bands to absorb the urine and keep my dog’s skin dry. I got these from Amazon. They tend to run small, so order a size bigger than you think you need, so you have plenty of overlap: