Dog Wetting Its Bed?

I have a Doberman who is 3 years old and he has recently started to wet the bed at night time, but it's not every night. We make him go outside to try and have a wee before bed, but this doesn't make a difference. The other night he woke at 5:00 am wanting to go for a wee which he pretty much has never done before. Could you give me some advice as to what it could be as the vet isn't very helpful?

By Victoria B