Preemie baby and breastfeeding

I have a preemie baby who is now thankfully home from the NICU.  He is breastfed but we have to do half our feedings with a bottle so he can get some nutritional supplements.  He doesn't seem to have a problem switching back and forth and latches on the boob great - but he never seems to get enough at the boob.   He either feeds for a long time (30-40 mins) or is hungry again in 1-2 hours.  With the bottle he goes 2-4 hours.  I know the bottle is easier for them, but is there any way to improve the strength of his suck or something to make breastfeeding more efficient for us?