does anyone know why an entire dog might be showing interest in a speyed bitch? She was speyed five months ago and he is sniffing her alot and showing lots of interest in her urine

It's not uncommon for speyed bitches to maintain a low level of interest for entire dogs.  Sometimes the interest ebs and flows as if she was in season as well.

If it's a recent change it's possible she has a UTI - dogs find the smell of infection very interesting. Best get a sample checked at the vet.

Having said that, it's also normal for dogs to be interested in urine, full stop!