Tickling is something that most humans cannot control their reaction to, and so takes away the bodily autonomy of the person that it is happening to, unless agreements have been made around consent, when it is okay, how to know when to stop if there is a time that it is okay.
There is PDA profile autism, where any loss of autonomy can make you immediately violent, or immediately dissociated, or immediately suicidal, without any capability to control that reaction, going all the way into dorsal vagal collapse and/or catatonia.
Tickling is often the first physical or sexual abuse that children experience, and can be highly triggering, again, possibly causing dorsal vagal collapse for people who had tickling that was legitimately used as torture, or that progressed to sexual assault.
There are many reasons why someone would not want to be tickled, and like with anything that removes someone’s bodily autonomy, doing so without someone’s consent is assault. Most people cannot control their physical reactions to tickling, and so when you assault them by tickling without consent, you have to be ready to receive whatever happens next, when the person can’t control that.
You assaulted him, and he defended himself. Tickling without consent is abuse.