He is so young.
They also get Separation Anxiety at various age junctures.
Not only when babies.
This is common.
Nothing 'wrong' with it.
My perspective is: I did what I needed to, per my kids' cues, for sleep.
And it they were going through various developmental changes/teething/hitting milestones/hitting Growth-spurts, I KNEW that... and I KNEW it tweaks a child's sleep.
I did not do cry it out or anything.
I, bonded with my kids, co-slept or gave feedings, as they needed, per their cues.
I knew, what they were crying for, per their sounds. I knew each sound they made.
Your baby is having Separation Anxiety. This is a developmental occurrence which is just, there. Part of development. It is INvoluntary.
It happens to all, babies and kids.
Also, sleep is NOT, static, in kids. Nor even in adults.
I don't know of any adult, that has slept the SAME way, since they were babies. All of us, when babies/toddlers/older children/pre-teens/teens/college kids/adults/elderly... ALL change in our sleep. Per life stages.
Sleep, will always change. It ebbs and flows.
It also depends on if he is napping.
Over-tired babies/kids... do not sleep well and actually they wake more and have a harder time sleeping. And for some, it actually makes them more 'hyper.'
This is but one stage, of sleep tweaks.
Just do what he needs, to fall asleep.
Then you can leave the room.
Kids, even adults, don't just pass out as soon as their heads hit the pillow. They need to, wind-down, first.
Also, don't do anything hyper or horse-play or super active, before naps or bedtime. That just keys up a child. Then they can't sleep. Because they were not wound-down, yet.
Pre-sleep and Pre-nap.... even 1 hour prior... make things QUIET and calm... and set the stage.
As a child gets older, they NEED to 'wind-down' first, before nap or bed. Their cognizance is different now... more aware. Of any little thing.
It is developmental.
I also do, when my kids got older, what Jennifer G. said... per my kids. I sit in the room. I read a magazine. I do not engage with them or even look at them. I tell them in a monotonous voice "sleep time..." and don't look or chat.
Or I do lie down by them, if need be.
They lull to sleep.
They like seeing me there. It is comforting. I have no problem with that.
THEN, I also say "I need to go pee... will come back..." and then I leave the room. And then I come back. But at varying intervals. But that, so my kids 'trust' that I will be back. Then I take longer and what not.
Then they fall asleep. Even if I am there or not.
It is a ROUTINE.
Also remember, that a child that age, does NOT even have, fully-developed Impulse Control yet. This is not fully developed yet, even until maybe 3 years old. For some kids, older.