A Short Sunderland or Consolidated Liberator would be the most likely options; Volume III of the official RAF history, The Fight Is Won lists the Order of Battle of Air Command, South East Asia in July 1944, the only aircraft employing dedicated Bomb Aimers that stand out are the Sunderland flying boats of 230 Squadron and the Liberators used for strategic bombing, reconnaissance, and special duties (dropping agents and supplies).

The RAF employed the vast majority of its standard heavy bombers (the Sterling, Halifax and Lancaster) over Europe, overseas commands were lower down the pecking order; the long range of the Liberator was also well suited to the considerable distances involved in South East Asia (most of the Liberator squadrons were based in India for their operations over Burma). There were still Wellingtons flying, the main bomber in SEAC until the arrival of the Liberator, but these had a Navigator acting as Bomb Aimer rather than a dedicated crewman.

If you'd like more specific information you may be able to request your relative's records - see the Guide to Requesting Military Records for more details.

Do you know which years he was in the Burma theatre?