How can I get my baby to sleep through the night?

My 1 year old keeps waking up in the middle of the night. Perfectly fine and clean diaper but still wont go back to sleep. Instead he cries. Yesterday he woke up at 2 am and cried on and off till 3 am. And this morning he woke up at 5:45am and wouldnt go back to bed till 6am then woke up when my husband went to work at 6:30 and kept waking up till around 7 am when he finally went to bed. (In the morning he sleeps in till 9 has been since he was born) he was sleep trained. Hes currently only taking one nap. Does anyone know why he would be waking up? He has 6 teeth but no others are showing and he doesnt act different during the day.