Answer:
He meant that Ben was happy and looking forward to all the fun he was going to have.
Step-by-step explanation:
To be light hearted means being happy and care free, and having high anticipation about something that a person is expecting positive outcomes out of that thing.
Therefor, in lines 15-17 of Tom Sawyer, Twain meant that Ben was happy and carefree, and he was excited and expecting to have a lot of fun.