student room problems in Leuven

Our apologies in advance for our lengthy post. 
It concerns a problem we are having with a student room which our son had rented in Leuven last September (not university owned accommodation).

The day he was supposed to move in last year September we discovered one of the walls was covered with mould. We spoke to the housing agency and they promised that they would treat the mould and repaint the room (the room had old chipped paint on the walls).  Several days later they said the room was ready to move in.

Our son moved in end of September and stayed until the end of May this year when he moved back home.  Before he left the room he paid a deposit and signed another contract for the same room for the academic year 2010/2011, beginning 20th September 2010.

On 13th September this year we went to his room to see if anything needed to be done before he moves in again. We again discovered mould on the same wall as before even though my son always kept his window ajar and the room was always well ventilated.  We asked the agency to treat the mould before our son moves in as our son had many respiratory problems last year.  They told us that they would fix this problem that same day.  Then on the 16th of September we went to the room again and saw that nothing had been done.  We went again to the agency and they told us that they could get their worker (a Polish man called Marius) only on Thursdays which meant that they would do it a week later, on September 23rd.  We agreed to wait but told them that they may not delay it beyond the 23rd as our son had to start with his semester. They promised they will do it on the 23rd.  When we went to the room again yesterday morning, nothing had been done yet.  So we went to the Leuven University housing services to ask them to speak to the housing agency on our behalf and to tell them that if they don't do the work today, we shall call someone ourselves and then send them the bill.  When university’s  housing services phoned the agency they said that their worker was in the room  already working on it.  They promised  that the wall will be treated for mould and not only painted over as last year.

So we went again to the room to see if what they told us was in fact the truth.  When we arrived to our son's room we found a man who had just finished painting over the mould with a thick coat of paint. When we asked him what did he do in the room he replied that he was told to paint only a coat of paint over the mould.  There was no mention of treating the mould at all.  In the meantime a representative from the agency also came. She didn’t know what to say or do. We told her to have the painting washed off with white spirit and if they didn’t have other products at hand to treat the mould with bleach and to do it today.  We demanded that she keeps her promise this time and to treat the mould as they were supposed to as the academic year had started and our son still had no room to move into.  She promised again.

However, we no longer have any confidence in this agency.  We would therefore  like to find out what legal steps can we take to see that the agency fulfills their obligation in providing a proper accommodation free from mold, and to claim back the expenses if they don’t finish the work by this week-end and we have to call in an expert to do the work.
We would be grateful for some advice.