Boomerang son

As far as I know.... you must register with the commune as soon as you arrive and you cannot claim unemployment benefit if you have not worked in Belgium. Low cost language training can usually be accessed via the commune if needed.

It might also be wise to register at a mutuelle ,or, ziekenfonds , when he needs medical care he is covered .

Unless he intends to stay less than 3 months, he must register within 8 days.
You should be able to register him with your mutuelle as your dependent - when I moved here my, then, girl-friend registered me with hers as I was unemployed at that point.

Even if not entitled to actual chomage payments -- it is worth registering as unemployed as that opens up a raft of free training courses