It sounds like he's gifted, though it's probably too early to begin worrying about HOW gifted. I wouldn't worry about the late walking... my daughter didn't walk until 13 months. I also wouldn't worry about his lower use of vocabulary, because of the whole trilingual thing. A number of studies of kids in bilingual families have demonstrated that it's pretty common for them to begin speaking late, because they're dealing with multiple sets of linguistic rules (sounds, construction, etc.) and expanded vocabulary... but once they do start talking, they advance very quickly in both languages, more than catching their peers. This is observed among non-gifted children. I would assume that a third language further complicates the language acquisition process, so just give him time.
The giveaways for giftedness in this story are his attention span, photographic memory, and the fact that he's already working towards rudimentary math and language skills that most kids aren't expected to start learning until kindergarten.
