Was he born at full-term (37 weeks or more)? If so his weight is fairly low, so perhaps they are checking him over for any complications that could be a result of possible intra-uterine growth retardation (IUGR). Or perhaps he was having some probs with breathing after his birth- sometimes babies struggle a little and breathe too quickly/shallowly and therefore do not have good oxygen saturation in their blood and so are brought into Special Care for monitoring, often in an incubator with pure oxygen pumped in. If this was the case they will also investigate any possible infection and monitor him for a few days (it takes 48hrs+ to culture blood samples to reveal any infection), and possibly have started him on intravenous antibiotics as a precaution.
Other possible reasons are severe jaundice which can require phototherapy which is usually undertaken in SCBU.
My 36 weeker (DD) weighed 5lb 8oz and needed a week in SCBU due to becoming "grunty" (fast, shallow breathing) after a couple of hrs post-birth. She was on a drip for 4 days (antibiotics and a drip to provide nutrients as she wasn't allowed to try milk for a couple of days), however she did not have an infection. It took a bit longer for her to get the hang of breastfeeding as a result of her breathing difficulties (she was very sleepy), and they couldn't let her out until she was able to feed well (instead of by tube-feeding which they started her on).