It may be purely developmental... and "separation anxiety". My son had this at about 6 months old. 'Separation Anxiety" crops up at certain ages... not just at one age.
Or teething, or gas, or he is just changing so much, as all babies do. It's not easy for them either.
And yes, baby sleep patterns DO change... and they change. Sleep is not static. Even adults go through bouts of trouble sleeping.
Try getting a crib toy...my son LOVES his Fisher Price "Ocean Wonders Aquarium." He turns it on himself, and it self-soothes him. ALso, my son has a stuffed cow that he LOVES to sleep with... it's comforting and a "pal" to sleep with. He gravitated to this cow on his own, and it quickly became his best sleeping pal. Or, there is the "Twilight Turtle Constellation Night Light." Both can be found on Amazon.
What I found, is that at this age, instead of plunking them down in the crib, and the crib being all bare and hard and uncomfortable... perhaps, (like we do) we have some baby safe stuffed toys in there with our son, and we put a soft blanket under his crib sheet to make is "softer" and more cozy. My son has enjoyed this and he now has a routine of playing with them a little before he is lulled to sleep, on his own. It self-soothes and is comforting for them. At least for my son... it worked, and it made his crib a LOT more enjoyable.
Also, listen to his cries... there are different cries... some are just that they are lonely, sometimes it IS distress, sometimes it is just complaining, sometimes it is calling for you, sometimes it is just the crying/fussying they do before actually falling asleep. IF you wake them just at that moment they are crying a little before sleeping... you will wake their cycle... and you will have to start all over again so that they can wind down and get sleepy again.
All babies are different... what works for one may not work for others. But find that one thing. Also, KEEP to a routine pre-sleep... and keep it consistent. SAME sleep time, same pre-sleep routine and process, SAME everyday. I've done this with both my kids... and although they go through developmental moments that will abrupt their sleep... it is a phase, and they will get back to their regular pattern. But, you have to be consistent in whatever method you choose. If you keep it up, they will attain good sleep habits later.
Also make sure he naps.... over tired babies/children actually do NOT sleep well at night.
ALso make sure that if this is a growth spurt, that he is not hungry. At growth spurts they get hungrier and more frequently. Breastmilk/formula is the PRIMARY source of nutrition for a baby in their first year of life. Feed him on demand.
Well have to go, my son just woke from his nap,
All the best,
Susan