Unless they have gone back on their word according to the CTC website the cost (if necessary to go to Qatar) will be covered by Qatar themselves so subject to clarification from CTC on that point you should not have to worry about paying for a flight.
I actually went to a selection day for Qatar but did not make it to stage 4. Salary details were given in the selection day presentation so unless they were missed off the day your son went he should have been told them. I forget off the top of my head the numbers but I thought they looked pretty good. The only potential downside is that obviously the line training wage is much reduced and there is no definite timescale for when line training will be complete - previous posts in the thread suggest that the CTC cadets who went to Qatar a while ago still have not completed.