Depends on the animal. You should be able to feel his pelt and figure there is space enough in there for another half Basenji. If not - yes, he is a tad overweight and so cut his rations.
Fat puppies are ok, but by 6 months old you want to get them slimmed down and kept at a lythe, svelte, correct weight for life.
Some pups mature size-wise very early and stop, other drag their heels and then put on a spurt. As other have suggested, what does your eye tell you ?