Well, they are switching dietitians, so I won't be seeing one until the new one starts at the end of October. In the meantime they told me what she'd tell me which is this:
With every meal put some fruit or veg or meat on their plate along with the food you know for sure they'll eat. But with the food they will eat to make it a smaller portion, so that they are still hungry, and so he'll perhaps try the other food on his plate. If he doesn't try it, fine, put him down and let him go play, don't make a big deal out of it, but don't give him more of the food he does like. So he's not starving.
So I am going to do this for a couple of weeks and see if it helps. I hope it does. I'll update this further if things change... **crossing fingers**