I would pack up the smaller stuff and have it shipped directly to him, rather than going through your parents and son - it's a simpler and cleaner way of doing it, and isn't going to involve making your family bear the burden of contacting him and arranging pickup, and storing it if they don't get in touch with him.
For the kayak - personally, I'd recommend doing the simplest thing that gets it out of your hair. Check out the Greyhound option, if it works, put it on the greyhound and contact your ex directly with pick up instructions. If that isn't an option, see if there is a used sporting goods place that will buy it, and send him the money via registered mail. If not, and you don't want to do Craigslist, then buy it off your ex and donate it somewhere, or trash it with a hammer, or anything that gets it out of your garage as soon as possible.
Two other things - check the local laws for abandoned possessions. Some areas have laws that regulate what you can do with stuff left at your place. And it might help a lot to make some appointments with a therapist with expertise in relationships. You have 13 years of practice of being in an unhealthy, on-again-off-again relationship, and that's a long pattern to break away from. Some expert help could go a long way in keeping you from getting sucked back in, and figuring out how to navigate stuff like the extra possessions in the most constructive way possible.