Stress caused itchy rash all over body doctor prescribed frenaler cort (10 tablets) rash came back once treatment was over. Can he take another 10 tabl?

Itchy rash. I suggest you see an allergist to see why you are developing this rash, steroids are not safe to take repeatedly, especially with high blood pressure. I would rather take an antihistamine such as Claritin (loratadine) Allegra or zyrtec to control the rash.
May not be stress. You cannot rely on cortisone alone to control your rash and should try to find an answer or safer treatment for the rash. Although stress can worsen any illness, it is difficult to prove that it is the only cause. As for another course of cortisone for a few days, it is likely safe but then what would you do if the rash comes back again?