I would definitely think 1st would be appropriate, especially given that he has completed Montessori K that tended more toward a 1st grade curriculum.
The one thing you might want to look at would be what the general makeup of students in your DS's new school is. For instance, do you live in a highly educated community where there would tend to be more bright to gifted students who might be at a similar level (although, with those scores, your DS isn't likely to have more than one or two intellectual peers), or do you live in a community where there is a lot wider range of kids' abilities and therefore are a lot of kids who are not at level? In classes where there is such a large range of abilities, teachers often have a hard time meeting the needs of the highest students because they have to focus so much on bringing the lower kids up. However, if there tend to be a lot of bright students, it might be easier for the teacher to differentiate in small groups.
FWIW, my DS is young for grade (Thanksgiving birthday with Sept 1 cutoff) and he had absolutely no problems at all (except that it was still too easy academically).
