My 45 Yr Old Son Is Experiencing Upper Lower Extremity

My 45 yr old son is experiencing upper lower extremity and left calf paresthesia------he has difficulty standing after sitting in a regular chair, easier to stand after sitting on a higher chair or stool-----he has a recent history ( 9 mos. approximately ) of falling----has been examined by his primary physician who states he has l leg weakness------he was examined in the ER a MRI 0f L spine was ordered-----Impression -no nerve root issue, some disc buldging, osteophyte, --he has psoriatic arthritis --diagnosed approx. 14 yrs. ago----I am concerned that he may have a serious disease `yet to be diagnosed