Apply for reconsideration. Send in a webform indicating the date of TRV expiry and that you had specified that your ticket was for a certain date yet given a TRV that expired a week earlier. I would ask for a TRV expiry date at least a week after your plane ticket to account for any flight delays. I would also consider contacting the embassy in your home country. The big issue is that it may take around a month to hear back from someone.
Your bf can also contact their MP to see if they can get your TRV expiry date changed to soon after your flight date. It is a very minor change so is a reasonable request. Your bf should get a reply from their MP about whether they are willing to contact IRCC within a week or two