I can't understand why so many people want yougsters to play ITF very soon.
I must admit I don't know Nikola family background but if he needn't earning that much of money on court to be able to pay his staff, there is no urge to play ITF.
Players reach their best at 25/27 (in most cases) and he is 18.
In such case like Nole, Djokovics have borrowed money to almost everybody around to pay trainers so I can understand they need money back but there are also plenty of players : Pernfors, Safin... that began playing at 19 or later on pro circuit.
It's not about money, almost all successful players today had very good results before they were 20, I think Isner started later, but he's an exception. Safin was insane talent, so he's not really a typical example. If you are 22 or so without any decent results, chances are you aren't going to have any.