Okay jurassic Park was my all time favourite series as a child (and is still a huge hyperfixation), but as a child JP3 was my all time favourite for some reason. But carrying on;
Headcanon that after 8 and a half weeks spent on isla sorna, after he was rescued, Eric Kirby turned into a total adrenaline junkie.
Of course, nothing compares to the feeling of running away from velociraptors, spinos, rexes, but he does his best to replicate the feeling.
Adrenaline can keep you alive even when your body is on the brink of exhaustion. How many times had Eric had to start running when out on a supply run? Or if he was asleep and the slightest noise made him wake up, afraid that there was a dinosaur outside trying to get in? Add sleep deprivation onto malnutrition, and adrenaline is a pretty handy friend to have.
In my hc, when he was finally home safe, he was more afraid of the absence of adrenaline than having it. So he started to do things to make his heart race. He picked fights, climbed the highest trees he could find, jumped out of his window in the middle of the night and raced through the streets, pretending there was a pack of velociraptors after him, (which worked a little too well, and an early worker found him crouched behind a dumpster at 6am having a PTSD attack). Even petty theft once, to have the security guard chase him. (Never again, his mom started crying. But he was desperate).
Anyway, Billy who was still in contact with the kirbys, heard all about it. His solution? Rock climbing. Base jumping. Bungee jumping. Anything, you name it. And when Eric is feeling a bit too ansty in the middle of the night, Billy will be there with his motorcycle, to take him on a joyride. It normally helps when Billy practically flies down residential streets. Eric argued that yeah, motorways are good in theory, but there are less things to dodge, which Billy just nodded to while thinking 'fuck this Kid needs a new therapist'.
(When Eric turns sixteen, Billy will buy him a dirt bike. And Amanda and Paul will drop to their knees in anguish, because that look in their sons eyes is downright devious).