It's easily possible he was a fighter pilot with another squadron during the period before transferring to bombers. In 1940, Canada had the lone fighter squadron operational as no.1 RCAF Squadron (Hurricane). But Canadian pilots were also posted to many RAF Squadrons.
In March 1941, no.1 RCAF become number 401. Henceforth, all RCAF fighter squadrons were numbered in the 4XX range. Canadians though, would contiue to fill rosters of RAF squadrons as well, some even becoming squadron leaders of RAF units.
But 153 Squadron is interesting because they began the war as a fighter unit, flying Defiants, followed by Bristol Beaufighter types. Is it not possible your uncle was stationed with this unit right from the start?
regards,
Jack