Since you are departing after the enter before date you will need to purchase a reentry permit prior to leaving Thailand if you wish to renter on this visa. You would need to depart and return prior to that enter before date in order to get an additional year. Your current visa will end on 16 December 2018. With your current travel plans you will not get any additional time on this visa.
Just be careful. You could end up paying when you go for your 90 day report and they see no 24 hour report in their system. That is the way the Korat immigration office deals with it.
You can enter and leave Thailand as often as you wish up to the enter before date. Even though you may have an extension that gives you permission to stay after that enter before date, if you decide to leave the country after that enter before date, you must, must, must purchase a reentry permit prior to departure.
he has applied for a visa and now seems to be asking what to do once he actually moves to Thailand. I am sure he will correct me if my assumption is incorrect.