Is he teething?
Is he getting enough intake? ie: breastmilk or Formula?
6 months of age is also a growth-spurt period in a baby. And at these junctures, many things go on, at the SAME time, in a baby and per development, and it can tweak their sleep. And at these times, they often need more intake and need more feedings/nursings.
At this age also, breastmilk/Formula is a baby's primary source of nutrition. Not solids. Solids is not as nutritionally dense, as breastmilk/Formula. And they need to be nursed on demand.
I know, a few women who had babies that would not sleep and was fussy. And the reason was: their baby was not getting enough intake. One woman did not have enough breastmilk, and the other just did not nurse her baby, on demand and her baby was "not getting fed enough... " in her words. And then, once she increased nursings, then her baby got better. And slept better. And the other woman fed her baby (who was on a bottle) when she, thought baby needed to be fed and she had her baby on a "schedule" for feedings. But so baby was not getting fed when the baby... needed it and was hungry.
Are you nursing your baby, when he wakes? And during anytime, on demand?
My kids as babies, had huge appetites. All day and all night long. And they grew like weeds. I always nursed before each nap and before sleep and when they woke. I knew if they were hungry, or just having wakings.
For a baby, sleeping all night long, is not the same as an adult who may sleep for 8 hours straight. For a baby, sleeping throughout the night, means even just for 4- 5 hours straight. And they do wake.
And if he is also teething, in conjunction with being hungry, a baby cannot sleep.