My 6wo gets so warm when he sleeps that he sweats, even when hes only in a light onsie! Is this notmal?

Can be. Some people are sweaty. If he's drinking well, peeing and pooping well, growing appropriately, and showing no other signs at all that he might be sick, then he's fine. At this age, though, if there is any question of illness at all, see your pediatrician. Better safe than sorry.
Babies sweat-yes. Babies are often very "warm natured" and they sweat easily especially around the head...The largest part of their body. If your 6 week old is eating well, gaining weight, happy, etc there is no cause for concern. Any time a parent is worried we are happy to see your child!