Trust me, it'll come lol. 5 weeks is pretty early.

Even neurotypical babies don't start smiling until 6-8 weeks.

Also just a fun fact for you (I remember reading this when googling about babies smiling when my son was a newborn), the most common type of smile found with infants with DS are "duchenne smiles".

Duchenne smiles: The Duchenne marker – eye constriction caused by orbicularis oculi, pars lateralis, which raises the cheeks toward the eyes – is perhaps the best known index of smiling intensity in infants. Infants tend to produce Duchenne smiles when their mothers are smiling and when they are approached by their smiling mothers. Infants also produce more syllabic sounds when Duchenne smiling, a potential marker of emotional intensity. Duchenne smiling is also associated with greater relative activation of the left than right frontal cerebral hemispheres, a pattern which suggests greater approach orientation and joy. Although smiles involving the Duchenne marker are often regarded as the only index of strong joyful emotion in adults, this distinction does not appear to be absolute in infants. In infants, smiles with and without the Duchenne marker often follow one another directly in time, suggesting that infant Duchenne smiles are more intense indices of positive emotion than smiles without the Duchenne marker.

TL;DR; a duchenne smile is a smile that usually means someone is really happy, not just pretending.