4seamer, welcome to the site. There have been other threads here that you can search regarding holding back. There is some very good information in these threads for both sides of the argument.
Some of the things you point out would tip the scale one way and some the other. You mention smaller frame parents and child as a reason to consider holding back. I would argue the opposite with that line item. If he is destined to be a small kid, an extra year won't change that. With his projected size, odds say he is less likely to successfully pursue high level baseball and, therefore, you would be less inclined to hold him back for athletic purposes. (Don't get me wrong - I am a big advocate of small players accomplishing big things but if you are considering the odds with hold-back decisions, it stands to reason you would have to consider the odds across the board.) Also, if he is academically ahead a year, holding him back would put him two years behind on getting started with his profession of choice and the salary that goes along with it.
LOTS to consider. Search those other threads. Be sure to enjoy the journey and make decisions based on what is best for him with or without baseball.