A.L.
I have a five month old with reflux and we have added cereal to his bottle because his doctor said it would help with spitup because of reflux. We ended up switching to AR formula which is thicker. It already has the rice starch added to it. When we added it to his bottle it was about 1 formula scoop to his 4-5 oz bottle. Just remember it will be thicker so you will need a different nipple for him to get it out. The only one we forund that worked was the platex dropins medium flo nipple. The rest of the bottle just have more holes in the nipple for faster flow. Playtex actully has one hole that gets bigger.
I would also try starting him on regular cereal or food. It might help fill him up before bed. We have tried starting my son on food for about a month. If they are ready they will take it and if not they will refuse so it is not too early to try.