Apologies for dragging this on, but just to update I have come to the conclusion that the animal guy is indeed right, supported by opinions of various others, incl. a Thai breeder on FB.
Hopefully he'll be this red some day
I also found out that local breeders say it can be more challenging and take more specific grooming to get a male Kamfa's kok to get very big -- not so automatic, like say a ZZ, so that could account for it being not so big now.
Thx all for input, the 'mystery' is basically solved (though I'm still not 100% sure it's a male or female), but I think male because no black markings on dorsal fin.

Hopefully he'll be this red some day

Thx all for input, the 'mystery' is basically solved (though I'm still not 100% sure it's a male or female), but I think male because no black markings on dorsal fin.