What Causes Pus Filled Bump On The Back?

my son pulled a tick off of his back saturday night. i am not sure how long it had been there before he pulled it off. i cleaned it out with alcohol when we got home. my daughter told me that yesterday the bump had pus in it. she cleaned it out with peroxide. do i need to take him to the dr. for a check up on this. last night the area was not red or pus filled.