There's very little specific grammar instruction in Rosetta Stone. That's actually something I don't like about it personally. I'm not sure whether that's a pro or a con for your son. I wouldn't call it memorization exactly either. The idea is that you learn language by using it, not by memorizing lists of vocab. You hear it, you see what it refers to (The Spanish words for "the girl is drinking" are next to a picture of a girl drinking water, for example), you say it, you remember it. It's supposed to be the way people learn their native language.
There is microphone use, and you can see a graph of your pronunciation. If it's not close enough to the correct pronunciation, you have to do it over to "pass" that part.
I have to note that the greatest help I had in understanding the specifics of English grammar and the names for terms like predicate nominative and subordinate clause was when I took a foreign language. I was much more aware of grammar when trying to put together sentences in a foreign language that had different rules.
I don't know how/if that interacts the 2E issues, but I'm not persuaded that learning Spanish will interfere with his learning grammar. I think I'd want some evidence, or at least a persuasive argument.
All of this is with the caveat that I'm not an expert, nor do I play one on TV.
