Please look at the photos in this article and give it a read. You can look at your duck from the same angles these photos were taken, but I suspect you have some nutritional issues going on.
This is common in ducklings even when given a high quality feed. They grow so fast and with Pekins being bred for meat production they grow even faster and their metabolism just can't handle everything the way it's meant to be sometimes.
The good news is that your duckling is still young and still growing and if treated throughout the rest of his growth he will hopefully balance out and the problem can be corrected!