Almost off Pred, but Very fatiqued

My husband is 80 and this is now his 2nd round with PMR. Had it for about 14 months, tapered the pred and it "went away". He was "PMR-free". for about a year. Then the PMR came back. He's been on a slow taper for about a year and is down to 1/2 mg of pred. every Other day. No pain at all, but the fatique and weakness seems to be taking over. Is this a symptom of not enough pred? or does it have to do with cortisol levels? We thank you all and Lord bless.