You say that he won't try cows milk or formula, but you haven't mentioned breast milk? Am I just being stupid or have you not tried expressing it?
I'm not sure what to suggest, and I really don't think there is a quick fix solution (otherwise we'd all be doing it!) If a baby doesn't want to sleep there aint much you can do in a short space of time.
How are you reacting when he's jumping all over the place and not sleeping? What is your routine? Can you see the point where it went from 6 to 3 hours, and did anything change at that point? Do you use a dummy? It could be sleep regression/growth spurt, in which case you may have to wait a long while for him to settle back in to a good routine again.
I'm sorry you're having a hard time, (everyone in this forum is so we understand!) I hope you manage to sort it out
Hello TSC - our paths have crossed on various sleep threads. We recently massively improved DD's (11 months) sleep (from 3-4 wake-ups to zero for last three nights!! by moving her into her own room. What about moving him into your 3 year old's room and have the three year old in with you for a bit? Give it a week - if he settles and sleeps better there then you can move the three year old back. If not then just him move him back, it;s just a week. We also night weaned DD. Didn;t want to do it by giving her Dad in the middle of the night suddenly made waking a lot less appealing. It's only been three nights I know but this is the biggest improvement we've seen since she was 4 months old.