Could be hes going through a growth spurt...
Elisa did that to me she slept through the night from 6-7 weeks and started waking back up at about 8 months when I went back to work which was a big change supposedly any drastic change can cause night terrors as well. Anyways got elisa back onto a schedule finally by 13 months and she started to sleep through the night again
could be a few different things though
~teething
~night terrors (afraid of you leaving)
~growth spurt
~ear infection
~sleeping too much during the day
~stimulating him too much during the day
a bunch of different reason rule out sickness first then work on the other ones...