Lizards and snakes are very sensitive to their surroundings. If you have such na expensive lizard, and one that is going to grow very large, you need to read up on your chosen pet! There are plenty of information websites out there, along with reptile forums, and plenty of books available!
All lizards need a heat lamp, not just the sun to keep their surroundings warm. If you don't have a lamp or a heating pad-they sell them in pet stores in the reptile section- you really need to go buy one.
Also, I suggest you head to the vet asap if you haven't already.
Other precautions to take so you don't pass along any germs, is to always wash your hands before and after holding your lizard. You can easily pass along bacteria that while safe for you, is harmful to him. Please do more research before you buy a reptile again.
I don't get to deal with reptiles very often - just the occasional weed wacked snake or a lost lizard. When they acted this way it was either being too cold or they were going to molt. Even our turtles will slow down when chilled.
Certainly your monitor could be sick but only a checkup by the vet will determine that. Check him for salmonella or e coli accidentally transfered to him. You might consider a doctor visit before it's too late.