Billy has had a lot of different haircuts. He's almost every year another haircut. This are the hairstyles he has/had:
This year Billy cut his hair, so it was spikey. He did this during filming Season 1 of Lab Rats, after filming 4 episodes. He's had this haircut before, when he was younger.
After filming Season 2 of Lab Rats, Billy let his hair grow longer and dyed it black. This was the first and only time he dyed his hair.
After finishing filming Season 3 of Lab Rats, Billy cut his hair really short. It's the same haircut he had as little kid for a while.