Wouldn't worry about feeding any more than you are now. He looks perfect!
Don't listen to the guinea pig or rabbit comment. Guinea pigs are not boa food and no need to bump up to rabbits. My female who is in the 6-7' range and about 15 lbs eats a 1 lb+/- rat every 2 weeks. Rather keep the boas growing slow and muscular than too fast where the develop excess fat. Boas are not meant to be retics. They seem to thrive when they are raised slower and kept thinner. A fat boa looks miserable, I can tell from the pics that he isn't underweight at all.