Guy keeps tickling me in my ribs...why? Is he flirting?

My brother is friends with him. He and I get along and have fun & hang out when he comes over but nothing much at school really. My brothers x-gf says that he told her he liked me. When she tried to get him to ask me out (I was there embarrassed as fuck) he said he didn't like me. That was months ago.
Anyways, he came up to me (behind me) the other day and jammed his fingers into my ribs (like on the sides) of my bra. He did this a couple times. A couple days later he did it again a few times. Then at lunch today he snuck up behind me and did it. Is he flirting or what's going on? I asked a friend who was a manager in his sports team if he was flirting and if he does this to other girls. She said he doesn't normally, and he was flirting. Idk. I like him so if he was flirting should I tickle him back? Is he flirting?
Guy keeps tickling me in my ribs...why? Is he flirting?
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