Welcome aboard, Matt! Disney is the perfect cruise line for your family with your young son! You are all sure to make some wonderful memories during your sailing!
No matter if you have a Port Adventure booked or not, all guests are welcome to disembark the ship at the ports of call. You might find that, depending on your destination, you could spend a few hours just exploring the port area on your own. Many of the ports have beaches nearby, but you didn't mention which sailing you're taking, so I can't give you specifics. However, please take a look at the Port Adventures that Disney offers in the
Family Adventures category. Many of these are available for all ages, and many go to a local beach at your destination. One of these adventures would be perfect for your young family, as you'll get Disney's service, the guarantee that the adventure is just right for the youngest cruisers, and most of these beach days are reasonably priced for families. Click your specific port and I'm sure you'll spot a relaxing beach trip that your entire family will enjoy. Bring along a few sand toys and you'll have a nice few hours with your little guy at the beach.
I hope you have a very magical cruise with your family, Matt. Please let me know if you have any other questions before you set sail, as I'd love to help!