The store doesn't sell gear that small. Phalanx may have some that small , not sure about 10p/Phalanx though.
I say this from experience. If you want him to wear 10p gear, have it screen printed yourself. At five tthe attention span isnt there for them to fully get into it. (For the most part) Ive had all 3 of my boys in jiu jitsu for 5+ years. The youngest being 5 when he started. Gear gets EXPENSIVE!!! As fast as they grow & tear shit up, you'll ask yourself" why did i buy a rashguard he wore four times? " lol