Hun my DD went through the exact same thing at 4 months old. All of a sudden she was waking hrly to 1.5hrly through the night. She did this every night for 6 weeks and then it stopped. She just decided one day to sleep through the night. Before that she was doing 3-4hrly wake ups, then it went to 1-1.5hrly for 6 weeks and now she sleeps through the night. We have the occassional crappy night but for the most she sleeps through. I didn't change anything either. It's nothing you've done. Just wanted to say there's hope and hopefully it's just a phase and he will be sleeping better in no time. I know how hard it is to function but for now do what you need to do, nap when he does to get through it because it will pass.
Sorry I can't offer any practical suggestions. Goodluck.