Hurling helmet size for 4 year old

My 4 and a half year old son has just started playing hurling. I need to get him a helmet. The smallest size in Elverys is Small. It seemed to fit ok to me, but I wouldn’t really know. The shop assistant suggested that I go online and get an extra small.
I’m happy to do that but a little bit concerned in case it is a bit too small for him, as I am presume that i won’t be able to return it.
Any advice from people who have bought helmets for a four year old?