Some of the usual spring staples in their dressers are basketball/athletic shorts, track pants, hoodies, long & short sleeved tees. Honestly, this doesn't change much from their winter wardrobe which will also include long thermal screen printed shirts, fleece lined track pants, jeans/cords and of course coats and beanies.
Some ideas I had for dresser filler are:
010 by seattlemom featuring adidas sandals
Boys don't have a lot of options like girls do but my boys know what they like and normally stick with it. My younger son, who is 8 1/2, is much more particular about what he wears while my older son still likes character tees and anything comfortable.