It's nursing behavior. Most kittens are removed from the litter much too early--they need mom until 8 weeks and the rest of the litter until 12 weeks. If he was removed from mom before 8 weeks he is still trying to get the comfort of nursing. Many cats removed too early never outgrow this behavior.
Eustache is a real cutie-pie---her markings are soooo adrorable....with the black nose and chin! And Moffat is going to be a gorgeous Brown Classic Tabby & White longhair. Lucky you! Nic pic of mommy too!