Grant は正確には投資かどうかではなく、オーナーの principal residence かどうかです。
息子さんがオーナーでその家に住んでいなければ適用できません。
Home owner grant について、BC州のサイトから一部抜粋します。
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To qualify for the grant:
You must
・Be the registered owner of the residence
・Be a Canadian citizen or permanent resident of Canada
・Live in B.C.
・Occupy the residence as your principal residence
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Your principal residence is the usual place that you make your home. It’s where you live and conduct your daily affairs, like paying bills and receiving mail, and it’s generally the residence used in your government records for things like your income tax, medical services plan, driver's licence and vehicle registration.