Italianmom,
I can't really comment on secondaire level here, child not there long enough, but so far so good, no-one has asked him if he is not Belgian, no-one has mocked him for his French, he is happy with his choice of school.
Homework in primaire was overwhelming at first for our then 10 year old coping in a new language, the younger one coped better as there was less homework. It depends very much on the school, from what I hear from other parents.
Doubling is common at all levels, maternelle, primaire, secondaire, not so much stigma as so many do it, schools appear to like to keep the class at same level, so it also means you can move up a year.
In the UK they would never do this, sometimes to the detriment of very bright or not bright.
Exams in primaire are obligatory in the French system in 2nd, 4th, 6th primary, the other years it is common in some schools to exams too, very school dependent. Same exams in every French school in 2nd, 4th, 6th and 6th exam called CERTIFICAT d ETUDES de BASE CEB and you need this CEB to go to mainstream secondaire, otherwise you have to find a French school which has a class for those who have not passed or taken this exam, and in a few secondaire schools there are special French immersion classes available theoretically only for certain nationalities, Italian not being one of these nationalities.
Schools in Wallonia are all French, in Flanders are all Dutch, except for 30 towns where a minority language has the right to an education in that language, eg German in Eupen, French in Kraainem.
Schools in Brussels are French or Dutch, but 80% of the Brussels population is French speaking, Dutch schools full of French speakers doing Dutch immersion and not recommended if your child speaks neither Dutch nor French. Many children in Flanders areas near Brussels sned their children to French schools in Brussels, as the French population is priced out of Brussels, they are moving further east, north and south into Flanders but wish their children to be educated still in French so they have to travel further to French schools in Brussels.
I'd say, if you choose to live in Flanders (unless in one Kraainem or Wezembeek-Oppem area), choose Dutch, if you choose to live in Wallonia, choose French, if Brussels again choose French.