I run a cut/sew shop specializing in marine canvas. I'm a one man shop but have worked for tyrants in the industry. Sounds like this guy is another dictator of his empire of dirt. Buy yourself a good walking foot machine and seize the means of production.
In all honesty, a simple box cushion should take about an hour and a half. I charge $100/hour. If I was to sub it out I have a sewer who would do it for $40/hour. I hate cushions with a passion and would tell the customer I was subbing it out but would still charge them $60/hour at least. Experience equals speed equals profit, and education is expensive. I'm sure you'll sew the next ones faster, but that doesn't mean you'll get paid extra to learn.
But fuck that guy. He will always pay the smallest amount possible and look for ways to keep his foot on your neck as you improve. Marine canvas isn't a simple trade to start in, but can be quite profitable if you don't fuck up. I encourage you to keep at it, but don't count on some asshole to help you.