Be glad your child has so many options. I've seen situations where this was a targeted area for severe cutbacks.
Chess club seems pretty self-explanatory, so it really just depends on whether your DS wants to play chess with other kids.
I'd want more information from the staff member/s leading the engineering club. What activities do they do?
I was often recruited to Academic Pentathlon teams or math competitions, which I hated. Who wants to spend a Saturday morning taking more tests? Quiz Bowl, on the other hand, was quite a lot of fun. It's a group activity, so there's the social benefit, and if your DS likes being competitive, I'd highly recommend it to him.
