TheFork users who ate in Leuven have said really good things about Troubadour, De Kloostergang and Point De Vue Brasserie. Maybe you should give them a try!
Some of the more popular Belgian restaurants having a terrace in Leuven according to TheFork users include:
Troubadour, with a 8.5 rating
De Kloostergang, with a 9.7 rating
Point De Vue Brasserie, with a 8.6 rating