dual immersion

DS knows Spanish and English. We speak English at home, and the dcp only
speaks Spanish. So, when it comes time for kindy in a few years, DS will
have the opportunity to be in a dual immersion program where 50% of the
classes are in English and 50% are taught entirely in Spanish. I like this
idea, but DH thinks DS will grow up to be only so-so in both languages. I
can see how the written language may suffer some, and it's only a
theoretical risk; but other than that, I don't see a problem. Has anyone
else had their child in a dual immersion program? What did you think? If
we don't do that, DS would still know Spanish, as it's spoken frequently in
this area, and I intend to hold onto the dcp for after school care. Would
it be better to keep him in an English only program until he's older, so
that his English reading and writing skills have more time to be ingrained?
He could always learn to read and write in Spanish in high school, though
maybe he won't do it as well if he learns it so late. This is a few years
down the line, but I'm always one to think ahead.