I wish they could tell you what the issue is! It could be teething. Have you tried any alternative pain relief methods like an amber teething necklace or even the Hyland teething tablets?

Another trick that might help is to try a cot toy that has lights and music. My DS2 went through a phase where he was waking up often o'night so I got him a Fisher Price Ocean Wonders aquarium cot toy and told him that it is his special toy just for bedtime. If he wakes up at night he is allowed to press the button and watch the toy (it has music, lights and bubbles in the water). This works really well.

I wish I had a magic solution for you! I hope sleep starts to improve for you both .