Oh damn, you're right. Would've been cool though.
A Warlock's spell slots at 3rd level are all 2nd level spell slots, so if a 1st level spell JP casts gains additional effects when cast at a higher level, it will always be cast at that higher level (phb p107).
This would have meant that his Armour of Agathys spell would have granted 10 temporary hit points, and deal 10 cold damage, instead of 5 apiece. <3