Did due diligence investigating soon after I saw that it didn't resolve on it's own at one YO, as I'd been told that it sometimes does. Had him to an opthamologist who told me not to be concerned. I was also thinking neurologist, but never investigated. Maybe Louie is part Siamese or Burmese? It is unilateral and not noticeable when the pupils are dilated. Only noticeable when they are constricted and you look at him head-on. Left alignment is about 5-10 degrees ectropic. His condition is likely congenital as he has had it ever since I first saw him at 12 weeks, altho possible that a problem occurred at or soon after birth.
No, it will not be because he is part-Siamese. The gene that causes strabismus in the Siamese breed needs two copies to have the effect it does, and hence the cat has to be colorpoint to be affected by the gene. I have an essay of an email somewhere from a lovely scientist Siamese breeder explaining in detail how this gene causes the problem..
The only reason I think it might be a good idea to consult a vet neurologist is that when I saw one this past week with my kitten she said that unilateral strabismus was more concerning than bilateral... but she also said that if it wasn't changing over months then it was less worrisome.