Hi Katherine,
There are button-style and slider-style MagicBand accessories available for purchase. You can see some available on
DisneyStore.com, but there are many, many more options sold in the Disney parks.
The accessories are all decorative. None of them have any role in helping keep the band secure. You're welcome to buy a slider for your son, but you should do so because you think it will make him happy, not because you think it will be functional.
In my experience, the MagicBands are quite secure. I've worn them on several trips and have never had any issues with them popping open. I've also had several friends use them with small kids and have not heard about any problems from them. If you're concerned, you could keep your son's Band in your pocket or purse and just pull it out when you need it.
In the unlikely event that he loses the Band, you can resolve the issue pretty easily. On the My Disney Experience app (phone or tablet) or on the DisneyWorld.com website, you can activate or deactivate any Band under your control. This way you can be sure that someone else does not use his functionality. Guest relations at the parks or the front desk of your hotel and help you get a replacement band.
Please let us know if you have additional questions. We're happy to help.