Ok I need some serious help!

Ds is 4.5 months. For the first 3 months of his life, he was a great sleeper. Almost immediately gave 3-4 hour stretches and eventually was up to a 7 hour stretch for the first part of the night.

Well, since 3 months, sleep has completely disintegrated! He is now waking every 1.5-2 hours and of course I start back to work now so I am seriously drained!

Some possible factors:
- around that time he grew out of the swaddle (his a&a blankets would wind up like inner tubes around his middle). Since then, I have tried a woombie, halo swaddles and regular sleep sacks but none seem to work. I have a Zipadee-zip on order.
- I started supplementing (and as of this week, nearly 75% of his diet) with formula. I thought formula would help with sleep though since he would be more full. I have tried several brands (including a gentle one as of this weekend) but don't notice a difference with any of them.

He still sleeps in our room in a halo bassinet.

Please please please someone help me get my good sleeper back (read in your most desperate possible voice!).