It depends; before or after OTL's surrender?
After would mean an earlier PoD to make him not surrender, which is very improbable in itself.
Before: the country is in a state of chaos, heir apparent Baudouin is 10, Parliament can't convene to elect a regent. Charles, count of Flanders, Leopold III's elder brother, probably takes the lead. He'll get along better with the government. The retreat from Belgium is more orderly and German progress is seriously slowed down. Many Belgian soldiers get evacuated from Dunkirk. A government-in-exile with the de facto Regent is set up in London. More resistance, pro-Nazi organisation less successful.
In the long run: no Question Royale after the war. The future of monarchy is still dubious... Belgium becomes more liberal than OTL. Earlier federalisation.