ah, here, i finally found the profile on the CAE
Chinese Algae Eater - Profile
minimum is twenty gallons on them due to their size, territorial needs, tendency towards eating tankmates, and I imagine their inflexibility (width of a tank should be at least twice as long as the adult size of a fish so that it can turn around easily). I understand that your son is attached to this fish, but he, most of all, simply can't stay in this tank. He WILL die if for no other reason than he is going to become stunted which will affect his health and shorten his lifespan.
If kept with ADFs you will eventually have no ADFs left (or at the verybest, eyeless ADFs)
However, you can keep some ADFs in this tank. they are small enough that you could probably keep three in this tank pretty safely, or maybe one and a little horde of shrimp. fish that could be in this tank, 1 betta, or maybe five male endler's livebearers, either of which should* be fine with ADFs or shrimp.
A nice set up for the tank might be 5 male endlers (females are larger, less colorful, and will result in tons of babies you haven't got room for

), 1 ADF, and maybe three ghost shrimp.
*bettas occasionally take offense to having roomates, but the endlers won't care at all.