Beckie,
I have a similar student at the moment, and have worked at a learning disability school in the past.
To help me manage my current “difficult” student, I actually met with her mum out of work to discuss her condition and tactics to handle it. It was very useful to come up with a plan of action, and now we’re pretty good at managing her outbursts.
Generally, I’ve found it’s all about personal connection.
Try and find a way to understand and bond with them even if it’s over something simple like a basic melody or song they enjoy. Then when things start getting out of hand try and steer it back to the melody/song and back to having fun. Try and distract with fun exercises, and don’t be afraid to go off the beaten track, since you’ll only find out what works with this person through trial and error.
The most important thing in my opinion though is trying to understand how they feel and finding common ground.
Hope this helps, and good luck. It’s very hard not to let a learning difficulty become a teaching difficulty!