we have had three litters with our cavalier of which we kept a male from last litter. Would appreciate any advice as thinking as offering him as a stud dog.
what are the cons to doing this.

When you accept a fee for the use at stud of your dog a contract is formed..remember that!
Think hard about if your dog is the sort that other people will want to use. The first thing is..is he healthy? I don't mean just evryday healthy I mean has he been tested for any heriditary problems associated with your breed..That costs money..so it may be that you invest in this dog's future as a Stud without knowing if you will get anything back.
For many breeders some success in the showring means that they are more confident about using your dog..If they don't see him at shows , where will they see him? How will they know he even exists?.If you don't show the market for your dog's services is limited .
Then... there is the contract...we live in a very agressive society nowadays..it's off to the solicitor if it all goes wrong. Some cases that have ended in tears and in court include...the stud dog owner did not ensure that the stud dog was fit for the purpose!..The stud dog owner did not take sufficient care of the visiting bitch whilst at stud!...Please take this warning to heart.
Then ...there are the problems of living with a dog who has 'had his oats'..It can be a life changing experience for some dogs...Territory marking becomes frequent , the dog may become more dominant to other kennel mates and his owner..Not always the case..but it is an area to consider.
Then...what are you going to do with visiting bitches...?...Some may have to stay with you for several days..Have you accomodation for a full in season bitch, away from all the other dogs?
Well, you did ask for the pitfalls...lol
It really isn't as simple as taking the money and letting them run!