It is developmentally appropriate for a 5 month old to not be sleeping through the night. Plus, it sounds like he is close, as 6 hours is considered sleeping through the night. Also, 6 months is a prime time for a growth spurt and it is common for a baby in that range to need another bottle in the night. While I am sure it seems torturous now, this will not last forever.
The addition of cereal or solids to a babies diet generally coincides with a baby's natural progression of lengthening sleep patterns at night. This is not a causal relationship. Adding more solids will not help your baby sleep. Honestly, only time will.
"There was no statistically significant trend or a consistent tendency of one group to have a higher proportion of sleepers than the other. Therefore, feeding infants rice cereal in the bottle before bedtime does not appear to make much difference in their sleeping through the night." NIH Medical Library - Study on Infant Sleep & Bedtime Cereal
Just as an aside -- cereal is not as nutrient packed as formula is, so feeding cereal is filling a babe's little tummy with less nutritious calories in terms of calcium, other minerals, fats, and protein.
As I said I know this is a hard sleepless time...could your husband and yourself alternate feedings so you are each getting the sleep you need?