working with Sen children

I work predominantly with a teenage boy in a wheelchair. I'm only 7 weeks pregnant, but because I'm so knackered, have hypermesis and constant dizziness I'm really struggling pushing him around. 
His classroom chair was a struggle before I was pregnant! Let alone now.
I have to change his nappy too, which involves bending, which I struggle with due to the dizziness. 
My work are crap! I've told them I'm struggling and they tell me that they can't do anything as there isn't anyone else! 
My GP tells me I shouldn't be lifting, hoisting, transferring or pushing him! Yet I'm expected to do all this. 
I don't see the miidwife until 4 weeks! 
I have already had a miscarriage and I'm terrified of another. But I'm stuck in the middle. 
Should I request a risk assessememt? Can myGP write a note saying I'm not to do those things?