Large Sulcata Is Not Using His Back Legs...Much?

(Simi Valley, CA USA)

Hello,

I have a large Sulcata that a woman gave me about 6 months ago. I am guessing, but I estimate his weight to be around 50 lbs. His shell is about 20 in long and 16" wide. When I got him he seemed to teeter from side to side like he was a little too heavy for his legs. In the past 6 weeks, I have noticed him using mostly his front legs to crawl on grass. When he is only pavement, he will use his back legs but not as much as he should to walk normally. His back legs do move and retract into his shell when I touch them. He has had a runny nose since I got him. His appetite seems decent, but he is often lethargic. His eyes also seem to water a little when it is really sunny outside or breezy.

I am thinking that he may have a calcium/D3 deficiency. Do you have any idea of what this can be. What can I do to help him? I live in Southern California in a desert like climate.