My ball python LOVES to wander around inside and outside of his enclosure. He's very curious but shy around a whole bunch of people.

Mine is about 5, I've heard (I adopted him from family who starved him), he looked just like yours too!
I thickened him up quickly by starting with mice fuzzies, small mice, small rats, and finally medium sized rats.
We still have a long way to go but we made progress and he looks (as a nearby petstore said when they saw him) 200% better than before.