I use a neoprene shorty wetsuit for my DDs in the local swimming pools - pays or itself after a while as it means they can stay in longer as they stay warmer (so we get more swimming for our money). We got our shorty from an ebay shop.
The shorty style are definitely far better than the vest style for keeping warm. Only concern for your beach holiday is that your DS might get too warm in a neoprene wetsuit of you are going somewhere warm. In the summer here my DDs were UV swimuits on the beach despite having circulatory problems that make them colder than most children would be.
btw - not tried 'babybans' but was talking to someone a few days ago who used them and said they don't stay on very well.
Also a few ideas for packing for the beach...
footwear (we use surfer style light plastic shoes) in case of weaver fish / glass /cigarettes on beach
hooded towel / coverall thing to keep warm if there is a breeze or for quick and easy drying
UV beach shelter (if you wish)- pop up style is best, esp with sandbags at the sides to weigh it down in the wind.
Spare hat - we always end up losing ours at some point and you can't be on the beach without one can you?
Spare swim nappy - I found reusable ones much more leak proof
Fresh water to rinse hands under for snacks
We always take a big bottle of squash which we freeze overnight
Identity tags to put mobile number on
Kite
My DDs have just turned 2and 4 and we spent most of last summer on our local beach - got used to taking just what we needed and no surplus (as I had to carry it all ).
Haven't mentioned snacks / suncream obvious things
Happy hols