If I'm not mistaken, Falabellas aren't a breed of their own and are still miniature horses. There is a very small population of them and those that do exist are registered, expensive, and VERY seldom if ever crossbred. Without being a pure falabella, he would still be considered 'just' a miniature, because as I said, falabellas are in fact miniatures that are merely from a certain lineage. They're built just like miniatures so really you couldnt just look at a horse and go 'oh, its mixed with a falabella', you'd have to go back to its pedigree :) falabellas are usually TOP quality as well and are kept that way to preserve worth, thus making a horse with more than one or two tiny conformational faults most likely not a falabella.
Check this site out. Falabella Miniature Horse Association - Official Site - FMHA
That being said, no. I don't think your horse is a falabella mix :) however he's very cute! What makes you think he's not pure mini? He looks like a typical grade miniature horse to me, and a very loved one at that. Sometimes they're the best kind because after all, they can't get all snooty about their bloodlines ;)