I recently noticed he wasn't using his back left leg when he walks 

He tucks it up in his shell. I haven't witnessed him hurting it. He still eating and drinking as normal and still looks himself. 

When he lays down he takes his leg out its only when he walks. 

Thanks in advance 😊

If it continues for more than a week, seek veterinary advice.

Meanwhile find your nearest 'exotic' vet.

Regards, 

Tom