Hi Siobhan, and welcome to the site!
I think it would be an *excellent* idea to neuter, for a number of reasons. I have two neutered two year old males, neutered at 10 months. In general, a neutered dog is calmer and more trainable, for males, less likely to mark indoors and display unwanted behaviours like humping legs, other dogs, toys etc; is at much lower risk for roaming (a male can smell a female in heat from over a mile away and will travel to find her); sharply lowers the risk of various male cancers, prevents unwanted litters of puppies that will most likely be out down as over 17,000 unwanted puppies and dogs were last year in Ireland at pounds alone. Managing an unnuetered dog has some big responsibilities as well, and this can be difficult for pet owners as he really needs to be kept under eye at all times.
*Sometimes* they are more prone to putting on weight as metabolism slows slightly -- so if you notice this happening, cut back on food about 20% or give more exercise. (Mine have never had this issue). Here's more info on why to neuter:
http://www.marvistavet.com/html/body_canine_neuter.html.
I'd have a concern though with a cavalier left outside to roam -- these are not a breed that should ever be left to wander by themselves off lead and without supervision. The reason why -- first, though I know a lot of pet owners don;t realise this, it is actually against Irish (and UK) law to allow a dog to roam in this way and they will be collected by wardens if sen on their own (I work with pound rescue groups and 90% of the dogs in pounds are strays collected by wardens, out "walking themselves". Very few are reclaimed because people think the dogs will eventually wander back. They don't -- the pound kills them after five days if they are unclaimed). I've seen a few cavaliers come in in this way and always make sure they at least come out and are rehomed -- few are reclaimed.
More importantly, AND I CANNOT STRESS THIS ENOUGH!!! -- cavaliers have NO road sense ATALL!! Most dogs do not really have "road sense" anyway -- this is definitely a myth -- but cavaliers in particular are bred to be "fearless" -- you can read it in the breed standard, which I have posted down in the FAQ section -- and therefore genetically, through centuries of breeding for this trait, they are disinclined to be afraid of something like a huge lorry or car bearing down on them (it also makes them wonderfully friendly and fearless of large dogs for example and therefore, not good watch dogs). Both of mine, as smart and easily trainable as they are, will walk directly into heavy traffic if given a chance -- so of course, they never are. Please reconsider allowing him ever outside to roam or most likely, you will not be able to enjoy his company for very long. This tendency to walk into traffic is endlessly remarked upon to me by people who own or who have owned cavaliers -- they should, like other toy breeds, be indoor dogs primarily and either allowed into a well fenced secure yard to play or taken for regular walks on lead (or off lead in a safe area, like a park or fields well away from any traffic). A lot of people do not realise how hopeless cavaliers are around traffic until they lose their cavalier in a road accident.
For example this is what one breed website says of cavaliers:
TEMPERAMENT:
VITAL, ENERGETIC, EBULLIENT, ACTIVE, LOVING, the Cavalier King Charles Spaniel is the perfect family dog. But it must be stressed, they are dogs that belong with people. They are not dogs to be left alone or housed outside as they thrive on human contact. Subsequently they are dogs that are most suited to family circumstances or for retired people. Apart from all their excellent qualities they do drop coat twice per year and have poor road sense so outside the well fenced family property, they should be kept on a lead.
And from another breed description site:
This breed sometimes displays a chasing instinct and should be kept well enclosed or leashed so he does not get lost or run over by a car!
And from the excellent dog breed website,
www.yourpurebredpuppy.com, in its "negatives" about the breed:
Chasing things that run. Cavalier King Charles Spaniels cannot be trusted off-leash. They have more spaniel (hunting dog) instincts than you might think. They will chase things that run and can easily end up under the wheels of a car.
I'm sure some others will want to talk about this too -- I know we have at least one board member who lost a cavalier in this very sad way and many who have had close shaves (I've had a few -- we all drop a lead, or open a door at the wrong moment, or have a dog run off sometime, no matter how careful we are. So I really know how scary that can be and how hopeless this breed is when confronted with road traffic).