Oh, Anna, sounds like you and Shrek are the victims of some bad advice. I'll try and get you started and see how it goes.
Bummer- if you were in the States I could tell you exactly where to get everything, but it sounds like you must be British- We can't get locust over here! I'll pm someone to help you locate things over there.
Two things you need right off- A
uvb bulb and a digital thermometer. They must have uvb to process vitamin D and calcium. A great bulb is the Repti-SUN 10.0, but ya'll have an equally good bulb over there that is easier for you to get. Again one of our friends "across the pond" will have to help find that. The temps should be between 100 and 110 farenheight on his basking and around 80 on the cool side. I know, I know, that's not how ya'll measure things. sorry I haven't lived in Europe since the '80s and have forgotten how to convert. You need a digital thermometer to get an accurite temp reading. Stick ons don't work well.
His feeders need to be dusted with calcium w/t D3 once a day 5 days a week and vitimins twice a week. He may need liquid calcium for a bit since he has started having probs. You could start feeding him phoenix worms (also called calci-grubs/worms) They are high in calcium naturally and dragons love them!
If you go to the top of the page, right under the banner, you will see Care sheets and articles. If you read through a few of those it will help you understand things a bit more. They helped me so much when I first started out. Check out the Beautiful Dragons
caresheet and food lists. They are color coded for the best and the worst things to feed your dragon. Some of us have printed them out and use them for reference!
You should probably get your little guy to a vet too. If you can afford it. He might need a calcium shot to give him a boost and stop those shakes. The other changes will still be needed to get him well.
I do wish you luck, keep us posted, please.
Sandy H