Usually when you have half of the pokedex filled he calls you and says "wow you have Pokemon not blah blah to johoto or kanto regions D:<
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Professor Elm will give you HM Rock Climb after the 16 badge.I think after you have collected all 16 badges you can talk to prof. elm and he will give it you.
he will give you the national pokedex then when you go to kanto he's in pallet town in his lab
It's not the "last" Legendary Pokemon that you can find in Pokemon HeartGold, but most people usually catch that one last given that you need to have both Kyogre and Groudon in your party for Professor Oak to give you the Jade Orb to get to Rayquaza.
After catching Kyogre, obtain a Groudon from a friend with Pokemon SoulSilver. You only need the Groudon temporarily. Talk to Professor Oak with both in your party, and Professor Oak will give you the Jade Orb to catch Raquayza.
He gives you the HM Rock Climb, and if you also defeat Red, you can get one of the Kanto Starter Pokemon.