Look at these pictures
People often put one of their lower legs and rest it on the thigh of the other leg.
Sometimes, that action is just his habit, but it could be that he want to relax when he does like that.
phrase of foot INFORMAL
take a rest, especially when reclining with one's feet raised and supported.
am not sure "put your feet up" is a good one to use in this case?
Note: I found this in the dictionary
cross–legged /ˈkrɑːsˌlɛgəd/ adjective Learner's definition of CROSS–LEGGED : having the legs crossed
She curled up into a cross-legged position.
— cross–legged adverb
She sat cross-legged.
The kids sat on the floor cross-legged. [=with their legs crossed and their knees spread wide apart]