Hi,

How old is your son and how well traveled is he so to say, additionally what cabin will he be traveling in?

1) Very open, DXB never sleeps. The time frame you specified is peak time, I'm talking 300+ arrivals/departures just for Emirates alone. There is a wide range of things to do to spend your time once through transit security and near your gate.
2) If EK will sell you that routing all on one ticket then you should not be worried as if there is a delay or your son fails to meet his connection due to any issue out of his control then EK will place him on the next available flight, my children ( boy and girl) have been flying independently since the age of 15 with EK and have never had any issues on their 30+ trips combined and i have full trust in the staff throughout the Emirates network to ensure their safety and comfort should something go wrong.
3) If he is purely transiting through DXB then it is just a trip to transit security then onward to his departing gate or a trip to his correct departing concourse etc. DXB can get pretty hectic at certain times of the day, but it is civilised hectic and nothing too overwhelming even for the adults!

I would personally go for the 75min connection as long as you and your son are happy with this, you seem to imply you will make use of the unaccompanied minor service (https://www.emirates.com/english/bef...ed-minors.aspx), if so you should not stress one bit. I as a parent who has gone through this many times thus i feel and understand your concerns however rest assured your son will love it, the crew and ground crew will go the extra mile along with the thought of knowing that the UAE and DXB airport is probably one of the best places
for a UM to be transiting through when flying Emirates.

I am sure others will have more to add