★.O.
What will help is to stop 'topping him off' with formula. By having him NOT nurse when cluster feeding's sole purpose is to stimulate your body to produce more - you are simply sabotaging your own supply.
Burping, rubbing the belly and block breastfeeding will help. Block breastfeeding is simply feeding from one breast for 2-3 feedings (within a 6 hours span) then moving on to the second side.
If you need something medical to make you feel more able to feed your child (which I'm sure you're doing just fine) is Fenugreek (enough per day to make you smell like maple syrup) combined with Blessed Thistle. Also, ensuring you are eating well, keeping hydrated and relaxed will greatly help too.