How old is your son and is he really going to fully dunk his head under the water?
Our ENT said that generally, the way ears are shaped, there is no need to use earplug if just splashing around in the bath or pool. But if your son is one who wants to jump off the edge and fully go under the water, then use the earplugs. We did get a pair, because our son has no fear and jumps right in the pool, and the ENT gave us a pair for $20. They can in a few sizes, and we just picked the most appropriate size. He did not tell us that we needed custom fit earplugs. I'll try to find the link (we ordered a second set online when one of the first set floated away in the pool). If I find it, I'll add to this post.
Here it is: http://www.proplugs.com/Merchant2/merchant.mv?Screen=PROD...
For reference, we ordered the extra small size for my 1 year old. By the time he turned 2, they kept falling out because they were too small. So for a 2 year old, I'd think you would need size 'medium small'. Not sure for a 5 year old. If you click around on the site, they have a request form to fill out to find someone in your area who will fit them for you. They are $13.95 online.