Boyfriend is H7 so the lost keys are turned H2 (signified by Jupiter). He needs to recheck his own possessions, or any area which is primarily his - Jupiter is in his own house. In Aquarius (an air sign) generally means 'off the floor' or 'high up'.

The other possible significator is the Moon. As you seem to live together, it's in House 11 - try guest beds or guest rooms. In a water sign suggests in or around soft things like fabric - bedding or in pockets is common.

Jupiter hasn't made any aspects since changing sign, but it's possible the Moon's last aspect to Mars is relevant, if it can show a person who may have picked them up - ask neighbours (boyfriend's turned House 3) or if parents have visited, ask them (boyfriend's turned House 10 is also your collective House 4) - all of these are Mars-ruled in the chart.