I have a 5 year old fixed male Min. Bull Dog. He has wet his own bed since he was a puppy. It doesn't matter if he's in a crate or an old towel next to our bed, or in the garage. We have a tile floored house so I feel he should have a rug somewhere to lie on. He's house trained and as long as there are no clothes or towels or bedding on the floor he's fine. He goes out side when he wants to. Gets praised when he pees. He checks out fine with the vet. Have tried to hold the water back at night. Never has an accident in the house. Just seems to want to mark his territory on his bed or anything he can find to lie on day or night. He just likes to lie in his own pee.

Have you actually seen him marking the bed and then laying down in it?

It is possible he is not doing it on purpose and in fact has urinary incontinence.
Some forms of incontinence only happen when the dog is asleep or very relaxed, the muscles do not work as well as they should so when the dog is not actually awake and aware they can leak.

Good news is there are drugs to help with the problem. I have a female poodle here who is on Propalin for her incontinence and have seen good results.

That makes sense except that when he is not sleeping on a bed or carpet he does not have any accidents. The answer to your question is no I have never seen him mark his bed but I see him laying in it every morning.

When this started happening, have you been washing his bedding? Or throwing it away?

Sometimes when dogs get fixed, it still won't stop them from marking. He could be marking his territory.

Unfortinately dogs don't think the same way we do. They don't mind laying in their musk (which is what scenting is). In fact, he probably enjoys smelling himself.

Try cleaning his bedding, the floor and everywhere else he has peed with an enzyme based solution, like Nature's Miracle. House hold cleaners just don't do a good enough job at killing the ammonia smell of urine. He can still smell it when you cant. See if that helps. If he continues to pee and/or mark after you have cleaned everything, I would look into male incontenance and see if there is anything out there to help you.