a 7days pass is enough
Currently the Routes east of Vienna in direction of Turkey are limited

A travelday goes from 00.00 till 23.59
In cases of Nighttrains counts the day of depature (when you board the train)

With just one travelday you can manage to go from Brussels depature at 6.25 with a change in Frankfurt to Vienna arrival at 16.45 (enough time to get food / drinks and have dinner) before catching the 19.42 Nighttrain to Bucharest. Arrival will be at 15.42 the nextday in Bucharest. Somewhere in Romania a dinning carriage will be added on the train.

In Bucharest you have then to stay one Night and catch then the 10.55 direct carrige to Istanbul :)

Just note the reservation Bucharest is bestwise booked in Romania.

Maybe you want to add Sofia as a stop over :)

From Sofia to Istanbul goes even a Nighttrain (this train later carry even the direct carriage from Bucharest to Istanbul) but the train from Sofia even have sleeper carriage and not just a Couchette like the carriage from Bucharest to Istanbul.

You need min. 2 Traveldays of Interrail to get there. But note train tickets in Romania,Bulgaria are quite cheap (compare them against Interrail) I usually travel with Interrail till Budapest or Bucharest and use then cheap tickets there.

For the return from Istanbul to Belgium you will need atleast 3traveldays means 7traveldays should be more than enough.

You may stop in Vienna (get two cities for the price of one and visit even Bratislava just 1hour away) another nice stop could be Budapest (maybe your son like trains and wanna explore the Children Rail in Budapest). Besides these major stops you may stop in few smaller Cities or places near lakes to get some rest from the hustle in Cities and on trains.

 

If you have any question dont hesitate to ask.

Or more detailed info (not specific for passholders) on seat61.com

The traditional route via old Yugoslavija is NO more -no trains over border there, only feasible via HU-RO.

Train to IST only runs summertime-last date not yet known.

Seewulf tells the fastest route with least nr of passdays-the pass is more meant to give you chance to make stops and see more on the way.