A change in your daytime routine could cause problems with sleep. He may be napping less during the day and therefore be overtired or he may be getting to a point where he needs to nap for a shorter period of time and he is not as tired a bedtime. Hopefully it is just a phase.
The child could very well be teething. That usually causes some pain which could be waking him up at night. Try ambesol or sucking on a piece of ice wrapped in a washcloth whild you are putting him to bed. He could ne going through separation anxiety which i sharder to fix. Wait it out.
Some young children have the tendancy to have what we call nightmares and this can be very upseting to them. They can wake up in the middle of the night and be terrified and not be able to go back to sleep or just dont want to be by themselves and have the comfort of their parents.