Why are they so interested in his bum?

I have a male cavalier king Charles spaniel, every time we meet anothe male dog on walks they are really interested in my dogs bum and will chase him around?

He really doesn't like it, he has his tail between his legs. One time I had to lift him up as the another dog grabbed hold of him and wouldn't let go! His owner ignored what was going on so I had to push the dog away

He's very friendly with all dogs and doesn't react in any way.

He is not castrated yet though will soon be!

If dogs have any problem with their anal glands it can cause a lot of interest from other dogs?

Many dogs routinely sniff each other's bottoms (and deposited faeces and urine) to get information about gender, health etc.

Most dogs are happy for a bit of mutual sniffing, including rear ends, by way of an introduction and then they move on to other things. Confident or relaxed dogs do not clamp their tails down, reducing scent transmission so curiosity is quickly and easily satisfied.

If he dislikes it, as some dogs do, the fact that he clamps his tail down and avoids them may be why they persist, because they haven't satisfied their curiosity?

Also to many dogs something interesting moving away automatically entices them to follow it.

Some males are more interested in assessing other males as potential competition.

The normal sniffing of bum he's ok with it's when they won't leave him alone? It's like they are trying to hump him?

He just wants to play but it's like they are trying to over power him! I don't want it to get to a stage where he gets attacked. If I hadn't lifted him up that time I think it would of turned badly I had to un clamp the dogs legs around him!

I hope when he's castrated it will stop! His anal glands are fine, he's had a vet check last week

OK, mounting attempts and lead up behaviours are very different to sniffing.

It is a lot pushier and lots of dogs are psychologically uncomfortable with it and it can be scarey if the dog is trapped and held by the other dog.

With a size difference it can also be physically painful due to weight, pressure, dragging and scraping claws, which sets the dog up to want to avoid it even more in future.

As some dogs can be bullies they can home in on easy or nervous targets.

Personally I would aim to block the other dog before it got to the point of having its legs clamped round him as this can make him even more uncomfortable at the prospect of it happening again.

Alternatively he may give up trying to avoid it as an unsuccessful strategy and discover that an offensive (aggressive) defence works better.

I would also aim to be more selective about his potential playmates to avoid him finding himself in these situations. In my experience negative social interactions can actually be harmful to effective socialisation.

Excellent advice Tass.

My dog is neutered and has too much interest from male dogs which he gets very defensive about. He wont allow another dog to sniff his rear end and when they persist, he gets very shirty.

It's now gotten to the point where he's warning other dogs away before they've even had a sniff and he can be very reactive so I agree, I would be careful about how this situation is handled and try not to let it escalate.