ACS and GEMS both have excellent reputations. My kids are at ACS and most of dh's colleagues live in Etihad Towers... We bucked the trend as we need more space. I think ACS and GEMS would both be do-able. ACS should be about 5-7 minutes. Oh, and don't stress, but do know that your son will be starting the second year of schooling here at any of the schools mentioned. The region typically starts students in KG1 at age 4. Just as an FYI ;).
You should make online applications to the schools you are interested in ASAP. GEMS World Academy is adding extra classes next year. You might have the best luck trying for a placement there.
edited to add: I sent you a pm
American Community School (ACS) is very near to Etihad Towers - probably a 5 minute drive (if you are not sure which apartments you will be in, ACS is even within walking distance of a lot of other high-rises on the Corniche - those that are near 30th and 32nd like the Al Hilal building and Crescent Towers). The other schools aren't particularly close to the Corniche. The on-island GEMS school would likely take you 20 minutes (*maybe* 15 depending on traffic). AISA would likely be about 20 minutes as well (further away than on-island GEMS - but faster roads). GEMS (the main off-island campus) would be 30-35-ish minutes. I'm not sure where Abu Dhabi International School is.
Good luck with the move!