He is simply changing.
Baby sleep, is not static.
It gets affected by:
1) growth-spurts
2) teething
3) cognitive changes and development
4) motor skills and development
5) Hitting major milestones
6) illness
7) hunger... especially at growth-spurt periods. Which, 6 months old is a growth-spurt time... AND a time in which a baby changes a lot developmentally. So, you need to feed on-demand, still. 24/7. Otherwise, his intake will not keep pace, with his growth/development and needs for more intake
8) Separation Anxiety
9) Object Permanence development
He is simply changing.
These junctures, in sleep, will occur and is normal.
Also, 'appetite' is never predictable...thus the reason for on-demand feeding, 24/7. Especially in the 1st year of life.
At times, a baby will even do what is called "Cluster feeding" in which they need to feed, even every single hour. Thus, you need to go by your baby's 'cues'..... for your baby.
ALL babies, will wake... and sleep patterns are not static. A baby, will not sleep the same as they did when a newborn, or when hitting developmental changes, or when at certain ages.... sure, there may be 'patterns' of sleep style... and it may be that way for a time... but it will change. At 1 years old, at 2 years old, and older.... then Teens and college kids change in sleep patterns too and in wakings.... even elderly go through that...
Your baby, is hungry.
He is growing.
He, if not already, will hit major milestones like crawling/sitting up/rolling over etc. And these things will affect their sleep. Temporarily. Because it is developmental based... you cannot "extinguish" it or 'make' them fall asleep... without waking.
"Sleeping through the night" for a baby... means, sleeping 5-6 hours straight. NOT all night, like an adult.
Also, if a baby is not napping and is over-tired... this only makes it more difficult for them to fall asleep and to stay asleep and to sleep well.
He should be napping, up to 3 times a day....
Also, if over stimulated before bed, a baby cannot sleep well nor fall asleep well.
**Also, he is going to bed too late.
My kids as babies that age, (I breastfed) woke up every 2-3 hours during the night. They were hungry. I knew their cues. They nursed me dry each time. I fed on demand for the 1st year. They had ginormous appetites.
For the 1st year, per our Pediatrician... breastmilk/Formula is a baby's PRIMARY source of nutrition.... not solids and not other liquids.
Also... feed him his Formula BEFORE solids... otherwise, a baby will simply be TOO full to drink... and will not get enough.
Even night time wakings... may still occur, but change.
It is never static..... it is per their many changes internally and cognitively... all at the same time, many times. Never really one thing exclusively...
all the best,
Susan