Deanna is right.
Also wanted to add, that both my kids did this too, at about the same age. It's not just your son.
You said you "are trying to think of something that changed...." But remember, it is not always just something in the environment that changes...it is also that THEY themselves are changing... and this in itself, can be the cause. It's all about ages and stages.
For me, and per our Pediatrician, babies and toddlers go through various stages of "separation anxiety" at different ages, and YES, even when they "should" go to bed and sleep. It's a phase and will pass.
Each child is different. And, at this age, they are changing so much... it's hard on them too. Not just us Parents. Comfort and soothe as you see best... ALSO keep in mind, kids/toddlers begin to get "fears" about night time or of the "dark." This too is developmental. They don't do it on purpose, to "control" us. At this age, and onward, they have "night terrors" and then later, "night mares." These are 2 different things. Both are developmental. Look it up online, and you will see lots of info. on it. And when he hits the 2's age... more will come. It's normal. But they need understanding.
ALL kids have different stages they go through and when they will sleep through the night. AND, sleep patterns are NOT static. They change. Even for adults, our sleep patterns vary and we don't sleep the same time, the same way, the same pattern every night. I don't think any grown-up has slept the SAME way since they were a baby. We all change, whatever the reason. A child is going to have "problems" sleeping here and there throughout their entire life, at various ages. Not just this age. At all ages. No child, and no age is "exempt." Each age has it's own "issues" and challenges. Sleep, is one of them. It not only makes us tired, it is for them too.
I think, personally, it is too young to put him in a toddler bed. And when it is time for this, this is another transition, and they have to get used to it. And by this time of toddler beds... they will climb out, dilly dally, want to come see you, play, and have time of difficulty sleeping too. So make sure, when and why you will need him to be in a toddler bed, and go according to his readiness....
Your son is also approaching 18 months old... this is another time of many developmental changes... and growth spurts, and their "emotions" are developing too. They are experiencing a lot more, and in different ways. It throws a kink into them, and in us, and we need to help navigate them and re-align our previous thoughts about them. Also, for some toddlers, from about 18 months old an onward.. .they are approaching 2 years old... and some kids, really go through changes at this time.
Maybe, try and get a book on the "2 year old" stage as well. Since this is coming up.
Just keep consistent and comfort him as you see fit. Kids need regularity to feel secure.
Does he get regular naps as well? If kids are over-tired or over-stimulated before sleeping, this also makes it hard for them to unwind and go to bed. My son gets clingy when he is tired, and just wants to stick to me like a crab on a rock.
Sorry for rambling, all the best,
Susan