What Are the Requirements for a "Jump" Bid Opposite a Takeout Double in Bridge?
I believe you're correct, and your partner is talking nonsense when he says "I thought your jump meant you had an opening hand".  Everything I can find on the subject suggests that the jump is appropriate for a hand with 8+ HCP, or perhaps 9-11 modified HCP as per this bridge tip from pro player Eddie Kantar:


  When responding to a takeout double, add one extra point for an unbid
  four card major and two extra points for an unbid five card major.
  Then if your total comes to 9-11 working points ( do not count
  anything for jacks or queens in any suit bid by an opponent) make a
  single jump response, not forcing. With less than 9 working points,
  make a non-jump response. With more than 11 revalued points, cuebid
  the opponent's suit.


With 8HCP and a diamond doubleton, you might be borderline, but you're still within your rights to test the waters.  Tell your partner (and opponents) to read up on responses to takeout doubles!